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<h1>Javascript Closures</h1>
<div id="faqNav">
  <a href="../../">FAQ</a> &gt; <a href="../">FAQ Notes</a>
</div>

<ul>
	<li><a href="#clIntro">Introduction</a></li>
	<li><a href="#clResO">The Resolution of Property Names on Objects</a>
		<ul>
			<li><a href="#clResA">Assignment of Values</a></li>
			<li><a href="#clResR">Reading of Values</a></li>
		</ul>
	</li>
	<li><a href="#clIRExSc">Identifier Resolution, Execution Contexts and scope chains</a>
		<ul>
			<li><a href="#clExCon">The Execution Context</a></li>
			<li><a href="#clScCh">scope chains and [[scope]]</a></li>
			<li><a href="#clIdRes">Identifier Resolution</a></li>
		</ul>
	</li>
	<li><a href="#clClose">Closures</a>
		<ul>
			<li><a href="#clAtGb">Automatic Garbage Collection</a></li>
			<li><a href="#clFrmC">Forming Closures</a></li>
		</ul>
	</li>
	<li><a href="#clClDo">What can be done with Closures?</a>
		<ul>
			<li><a href="#clSto">Example 1: setTimeout with Function References</a></li>
			<li><a href="#clObjI">Example 2: Associating Functions with Object Instance Methods</a></li>
			<li><a href="#clEncap">Example 3: Encapsulating Related Functionality</a></li>
			<li><a href="#clOtE">Other Examples</a></li>
		</ul>
	</li>
	<li><a href="#clAc">Accidental Closures</a></li>
	<li><a  href="#clMem">The Internet Explorer Memory Leak Problem</a></li>
</ul>

<h2 id="clIntro">Introduction</h2>

<blockquote cite="http://groups.google.com/groups?selm=wu535hos.fsf@hotpop.com">
	<dl>
		<dt id="clDefN">Closure</dt>
		<dd>A &quot;closure&quot; is an expression (typically a function) that
		    can have free variables together with an environment that binds
		    those variables (that &quot;closes&quot; the expression).
		<dd>
	</dl>
</blockquote>

<p>
Closures are one of the most powerful features of ECMAScript
(javascript) but they cannot be property exploited without
understanding them. They are, however, relatively easy to create,
even accidentally, and their creation has potentially harmful
consequences, particularly in some relatively common web browser
environments. To avoid accidentally encountering the drawbacks and
to take advantage of the benefits they offer it is necessary to
understand their mechanism. This depends heavily on the role of
scope chains in identifier resolution and so on the resolution of
property names on objects.
</p>

<p>
The simple explanation of a Closure is that ECMAScript allows inner
functions; function definitions and function expressions that are
inside the function bodes of other functions. And that those inner
functions are allowed access to all of the local variables, parameters
and declared inner functions within their outer function(s). A closure
is formed when one of those inner functions is made accessible outside
of the function in which it was contained, so that it may be executed
after the outer function has returned. At which point it still has
access to the local variables, parameters and inner function
declarations of its outer function. Those local variables, parameter
and function declarations (initially) have the values that they had
when the outer function returned and may be interacted with by the
inner function.
</p>

<p>
Unfortunately, properly understanding closures requires an
understanding of the mechanism behind them, and quite a bit of
technical detail. While some of the ECMA 262 specified algorithms have
been brushed over in the early part of the following explanation, much
cannot be omitted or easily simplified. Individuals familiar with
object property name resolution may skip that section but only people
already familiar with closures can afford to skip the following
sections, and they can stop reading now and get back to exploiting
them.
</p>

<h2 id="clResO">The Resolution of Property Names on Objects</h2>

<p>
ECMAScript recognises two categories of object, &quot;Native Object&quot;
and &quot;Host Object&quot; with a sub-category of native objects called
&quot;Built-in Object&quot; (ECMA 262 3rd Ed Section 4.3). Native objects
belong to the language and host objects are provided by the environment,
and may be, for example, document objects, DOM nodes and the like.
</p>

<p>
Native objects are loose and dynamic bags of named properties (some
implementations are not that dynamic when it comes to the built in
object sub-category, though usually that doesn't matter). The defined
named properties of an object will hold a value, which may be a
reference to another Object (functions are also Objects in this sense)
or a primitive value: String, Number, Boolean, Null or Undefined. The
Undefined primitive type is a bit odd in that it is possible to assign
a value of Undefined to a property of an object but doing so does not
remove that property from the object; it remains a defined named
property, it just holds the value <code>undefined</code>.
</p>

<p>
The following is a simplified description of how property values are
read and set on objects with the internal details brushed over to the
greatest extent possible.
</p>

<h3><a name="clResA" id="clResA">Assignment of Values</a></h3>

<p>
Named properties of objects can be created, or values set on existing
named properties, by assigning a value to that named property. So
given:-
</p>

<pre>
var objectRef = new Object(); <span class="commentJS">//create a generic javascript object.</span>
</pre>

<p>
A property with the name &quot;testNumber&quot; can be created as:-
</p>

<pre>
objectRef.testNumber = 5;
<span class="commentJS">/* - or:- */</span>
objectRef[&quot;testNumber&quot;] = 5;
</pre>

<p>
The object had no &quot;testNumber&quot; property prior to the
assignment but one is created when the assignment is made. Any
subsequent assignment does not need to create the property, it just
re-sets its value:-
</p>

<pre>
objectRef.testNumber = 8;
<span class="commentJS">/* - or:- */</span>
objectRef[&quot;testNumber&quot;] = 8;
</pre>

<p>
Javascript objects have prototypes that can themselves be objects, as
will be described shortly, and that prototype may have named
properties. But this has no role in assignment. If a value is assigned
and the actual object does not have a property with the corresponding
name a property of that name is created and the value is assigned to
it. If it has the property then its value is re-set.
</p>

<h3><a name="clResR" id="clResR">Reading of Values</a></h3>

<p>
It is in reading values from object properties that prototypes come
into play. If an object has a property with the property name used in
the property accessor then the value of that property is returned:-
</p>

<pre>
<span class="commentJS">/* Assign a value to a named property. If the object does not have a
   property with the corresponding name prior to the assignment it
   will have one after it:-
*/</span>
objectRef.testNumber = 8;

<span class="commentJS">/* Read the value back from the property:- */</span>

var val = objectRef.testNumber;
<span class="commentJS">/* and  - val - now holds the value 8 that was just assigned to the
   named property of the object. */</span>
   </pre>

<p>
But all objects may have prototypes, and prototypes are objects so they, in
turn, may have prototypes, which may have prototypes, and so on forming
what is called the prototype chain. The prototype chain ends when one
of the objects in the chain has a null prototype. The default prototype for the
<code>Object</code> constructor has a null prototype so:-
</p> 

<pre>
var objectRef = new Object(); <span class="commentJS">//create a generic javascript object.</span>
</pre>

<p>
Creates an object with the prototype <code>Object.prototype</code> that itself has a
null prototype. So the prototype chain for <code>objectRef</code> contains only one
object: <code>Object.prototype</code>. However:-
</p>

<pre>
<span class="commentJS">/* A &quot;constructor&quot; function for creating objects of a -
   MyObject1 - type.
*/</span>
function MyObject1(formalParameter){
    <span class="commentJS">/* Give the constructed object a property called - testNumber - and
       assign it the value passed to the constructor as its first
       argument:-
    */</span>
    this.testNumber = formalParameter;
}

<span class="commentJS">/* A &quot;constructor&quot; function for creating objects of a -
   MyObject2 - type:-
*/</span>
function MyObject2(formalParameter){
   <span class="commentJS">/* Give the constructed object a property called - testString -
      and assign it the value passed to the constructor as its first
      argument:-
    */</span>
    this.testString = formalParameter;
}

<span class="commentJS">/* The next operation replaces the default prototype associated with
   all MyObject2 instances with an instance of MyObject1, passing the
   argument - 8 - to the MyObject1 constructor so that its -
   testNumber - property will be set to that value:-
*/</span>
MyObject2.prototype = new MyObject1( 8 );

<span class="commentJS">/* Finally, create an instance of - MyObject2 - and assign a reference
   to that object to the variable - objectRef - passing a string as the
   first argument for the constructor:-
*/</span>

var objectRef = new MyObject2( &quot;String_Value&quot; );
</pre>

<p>
The instance of <code>MyObject2</code> referred to by the <code>objectRef</code> variable has a
prototype chain. The first object in that chain is the instance of
<code>MyObject1</code> that was created and assigned to the prototype
property of the <code>MyObject2</code> constructor. The instance of 
<code>MyObject1</code> has a prototype, the object that was assigned to the function
<code>MyObject1</code>'s prototype property by the implementation. That object has
a prototype, the default <code>Object</code> prototype that corresponds with the
object referred to by <code>Object.prototype</code>. <code>Object.prototype</code> has a null
prototype so the prototype chain comes to an end at this point.
</p>
<p>
When a property accessor attempts to read a named property form the
object referred to by the variable <code>objectRef</code> the whole
prototype chain can enter into the process. In the simple case:-
</p>

<pre>
var val = objectRef.testString;
</pre>

<p>
- the instance of <code>MyObject2</code> referred to by <code>objectRef</code> has
a property with the name &quot;testString&quot; so it is the value of
that property, set to &quot;String_Value&quot;, that is assigned to the
variable <code>val</code>. However:-
</p>

<pre>
var val = objectRef.testNumber;
</pre>

<p>
- cannot read a named property form the instance of
<code>MyObject2</code> itself as it has no such property but the 
variable <code>val</code> is set to the value of <code>8</code> rather
than undefined because having failed to find a corresponding named
property on the object itself the interpreter then examines the object
that is its prototype. Its prototype is the instance of
<code>MyObject1</code> and it was created with a property named
&quot;testNumber&quot; with the value <code>8</code> assigned to that property, so
the property accessor evaluates as the value <code>8</code>. Neither
<code>MyObject1</code> or <code>MyObject2</code> have defined a
<code>toString</code> method, but if a property accessor attempts to
read the value of a <code>toString</code> property from
<code>objectRef</code>:-
</p>

<pre>
var val = objectRef.toString;
</pre>

<p>
- the <code>val</code> variable is assigned a reference to a function.
That function is the <code>toString</code> property of
<code>Object.prototype</code> and is returned because the process of
examining the prototype of <code>objectRef</code>, when
<code>objectRef</code> turns out not to have a &quot;toString&quot;
property, is acting on an object, so when that prototype is found to
lack the property its prototype is examined in turn. Its prototype
is <code>Object.prototype</code>, which does have a
<code>toString</code> method so it is a reference to that function
object that is returned.
</p>

<p>
Finally:-
</p>

<pre>
var val = objectRef.madeUpProperty;
</pre>

<p>
- returns <code>undefined</code>, because as the process of working up the prototype
chain finds no properties on any of the object with the name
&quot;madeUpPeoperty&quot; it eventually gets to the prototype of
<code>Object.prototype</code>, which is null, and the process ends
returning <code>undefined</code>.
</p>

<p>
The reading of named properties returns the first value found, on the
object or then from its prototype chain. The assigning of a value to a
named property on an object will create a property on the object itself
if no corresponding property already exists.
</p>

<p>
This means that if a value was assigned as
<code>objectRef.testNumber = 3</code> a &quot;testNumber&quot; property
will be created on the instance of <code>MyObject2</code> itself, and
any subsequent attempts to read that value will retrieve that value as
set on the object. The prototype chain no longer needs to be examined
to resolve the property accessor, but the instance of
<code>MyObject1</code> with the value of <code>8</code> assigned to its
&quot;testNumber&quot; property is unaltered. The assignment to the
<code>objectRef</code> object masks the corresponding property in its
prototype chain.
</p>

<p>
Note: ECMAScript defines an internal <code>[[prototype]]</code>
property of the internal Object type. This property is not directly accessible with
scripts, but it is the chain of objects referred to with the
internal <code>[[prototype]]</code> property that is used in property
accessor resolution; the object's prototype chain. A public
<code>prototype</code> property exists to allow the assignment,
definition and manipulation of prototypes in association with the
internal <code>[[prototype]]</code> property. The details of the
relationship between to two are described in ECMA 262 (3rd edition)
and are beyond the <dfn>scope</dfn> of this discussion.
</p>

<h2 id="clIRExSc">Identifier Resolution, Execution Contexts and scope chains</h2>

<h3 id="clExCon">The Execution Context</h3>

<p>
An <dfn>execution context</dfn> is an abstract concept used by the ECMSScript
specification (ECMA 262 3rd edition) to define the behaviour required
of ECMAScript implementations. The specification does not say anything
about how <dfn>execution contexts</dfn> should be implemented but execution
contexts have associated attributes that refer to specification defined
structures so they might be conceived (and even implemented) as objects
with properties, though not public properties.
</p>

<p>
All javascript code is executed in an <dfn>execution context</dfn>. Global code
(code executed inline, normally as a JS file, or <span class="initialism" title="HyperText Mark-up Language"><abbr title="HyperText Mark-up Language">HTML</abbr></span> page, loads) gets
executed in <dfn>global</dfn> <dfn>execution context</dfn>, and each invocation of a function (possibly as a constructor) has an associated
<dfn>execution context</dfn>. Code executed with the <code>eval</code> function
also gets a distinct execution context but as <code>eval</code> is
never normally used by javascript programmers it will not be discussed
here. The specified details of <dfn>execution contexts</dfn> are to be found in
section 10.2 of ECMA 262 (3rd edition).
</p>

<p>
When a javascript function is called it enters an <dfn>execution context</dfn>,
if another function is called (or the same function recursively) a new
<dfn>execution context</dfn> is created and execution enters that context for the
duration of the function call. Returning to the original execution
context when that called function returns. Thus running javascript code
forms a stack of <dfn>execution contexts</dfn>.
</p>

<p>
When an <dfn>execution context</dfn> is created a number of things happen in a
defined order. First, in the <dfn>execution context</dfn> of a function, an
&quot;Activation&quot; object is created. The activation object is
another specification mechanism. It can be considered as an object
because it ends up having accessible named properties, but it is not a
normal object as it has no prototype (at least not a defined prototype)
and it cannot be directly referenced by javascript code.
</p>

<p>
The next step in the creation of the <dfn>execution context</dfn> for a function
call is the creation of an <code>arguments</code> object, which is an
array-like object with integer indexed members corresponding with the
arguments passed to the function call, in order. It also has
<code>length</code> and <code>callee</code> properties (which are not
relevant to this discussion, see the spec for details). A property of
the Activation object is created with the name &quot;arguments&quot;
and a reference to the <code>arguments</code> object is assigned to
that property.
</p>

<p>
Next the <dfn>execution context</dfn> is assigned a <dfn>scope</dfn>. A <dfn>scope</dfn> consists of a
list (or chain) of objects. Each function object has an internal
<code>[[scope]]</code> property (which we will go into more detail
about shortly) that also consists of a list (or chain) of objects.
The <dfn>scope</dfn> that is assigned to the <dfn>execution context</dfn> of a function call
consists of the list referred to by the <code>[[scope]]</code> property
of the corresponding function object with the Activation object added
at the front of the chain (or the top of the list).
</p>

<p>
Then the process of &quot;variable instantiation&quot; takes place using an object
that ECMA 262 refers to as the &quot;Variable&quot; object. However,
the Activation object is used as the Variable object (note this, it is
important: they are the same object). Named properties of the Variable
object are created for each of the function's formal parameters, and if
arguments to the function call correspond with those parameters the
values of those arguments are assigned to the properties (otherwise the
assigned value is <code>undefined</code>). Inner function definitions
are used to create function objects which are assigned to properties of
the Variable object with names that correspond to the function name
used in the function declaration. The last stage of variable
instantiation is to create named properties of the Variable object
that correspond with all the local variables declared within the
function.
</p>

<p>
The properties created on the Variable object that correspond with
declared local variables are initially assigned <code>undefined</code>
values during variable instantiation, the actual initialisation of
local variables does not happen until the evaluation of the
corresponding assignment expressions during the execution of the 
function body code.
</p>

<p>
It is the fact that the Activation object, with its
<code>arguments</code> property, and the Variable object, with named
properties corresponding with function local variables, are the same
object, that allows the identifier <code>arguments</code> to be treated
as if it was a function local variable.
</p>

<p>
Finally a value is assigned for use with the <code>this</code> keyword.
If the value assigned refers to an object then property accessors
prefixed with the <code>this</code> keyword reference properties of
that object. If the value assigned (internally) is null then the
<code>this</code> keyword will refer to the global object.
</p>

<p>
The global execution context gets some slightly different handling as
it does not have arguments so it does not need a defined Activation
object to refer to them. The global execution context does need a <dfn>scope</dfn>
and its <dfn>scope chain</dfn> consists of exactly one object, the global object.
The global execution context does go through variable instantiation,
its inner functions are the normal top level function declarations that
make up the bulk of javascript code. The global object is used as the
Variable object, which is why globally declared functions become
properties of the global object. As do globally declared variables.
</p>

<p>
The global execution context also uses a reference to the global object
for the <code>this</code> object.
</p>

<h3 id="clScCh">scope chains and [[scope]]</h3>

<p>
The <dfn>scope chain</dfn> of the execution context for a function call is
constructed by adding the execution context's Activation/Variable
object to the front of the <dfn>scope chain</dfn> held in the function
object's <code>[[scope]]</code> property, so it is important to
understand how the internal <code>[[scope]]</code> property is
defined.
</p>

<p>
In ECMAScript functions are objects, they are created during variable
instantiation from function declarations, during the evaluation of
function expressions or by invoking the <code>Function</code>
constructor.
</p>

<p>
Function objects created with the <code>Function</code> constructor
always have a <code>[[scope]]</code> property referring to a <dfn>scope
chain</dfn> that only contains the global object.
</p>

<p>
Function objects created with function declarations or function
expressions have the <dfn>scope chain</dfn> of the execution context in which
they are created assigned to their internal <code>[[scope]]</code>
property.
</p>

<p>
In the simplest case of a global function declaration such as:-
</p>

<pre>
function exampleFunction(formalParameter){
    ...   <span class="commentJS">// function body code</span>
}
</pre>

<p>
- the corresponding function object is created during the variable
instantiation for the global execution context. The global execution
context has a <dfn>scope chain</dfn> consisting of only the global object. Thus
the function object that is created and referred to by the property of
the global object with the name &quot;exampleFunction&quot; is
assigned an internal <code>[[scope]]</code> property referring to a
<dfn>scope chain</dfn> containing only the global object.
</p>

<p>
A similar <dfn>scope chain</dfn> is assigned when a function expression is
executed in the global context:-
</p>

<pre>
var exampleFuncRef = function(){
    ...   <span class="commentJS">// function body code</span>
}
</pre>

<p>
- except in this case a named property of the global object is created
during variable instantiation for the global execution context but the
function object is not created, and a reference to it assigned to the
named property of the global object, until the assignment expression is
evaluated. But the creation of the function object still happens in the
global execution context so the <code>[[scope]]</code> property of the
created function object still only contains the global object in the
assigned scope chain.
</p>

<p>
Inner function declarations and expressions result in function objects
being created within the execution context of a function so they get
more elaborate scope chains. Consider the following code, which defines
a function with an inner function declaration and then executes the
outer function:-
</p>

<pre>
function exampleOuterFunction(formalParameter){
    function exampleInnerFuncitonDec(){
        ... <span class="commentJS">// inner function body</span>
    }
    ...  <span class="commentJS">// the rest of the outer function body.</span>
}

exampleOuterFunction( 5 );
</pre>

<p>
The function object corresponding with the outer function declaration
is created during variable instantiation in the global execution context
so its <code>[[scope]]</code> property contains the one item scope
chain with only the global object in it.
</p>

<p>
When the global code executes the call to the
<code>exampleOuterFunction</code> a new execution context is created for
that function call and an Activation/Variable object along with it.
The <dfn>scope</dfn> of that new execution context becomes the chain consisting of
the new Activation object followed by the chain refereed to by the
outer function object's <code>[[scope]]</code> property (just the
global object). Variable instantiation for that new execution context
results in the creation of a function object that corresponds with the
inner function definition and the <code>[[scope]]</code> property of
that function object is assigned the value of the <dfn>scope</dfn> from the
execution context in which it was created. A <dfn>scope chain</dfn> that contains
the Activation object followed by the global object.
</p>

<p>
So far this is all automatic and controlled by the structure and
execution of the source code. The <dfn>scope chain</dfn> of the execution context
defines the <code>[[scope]]</code> properties of the function objects
created and the <code>[[scope]]</code> properties of the function
objects define the <dfn>scope</dfn> for their execution contexts (along with the
corresponding Activation object). But ECMAScript provides the
<code>with</code> statement as a means of modifying the scope chain.
</p>

<p>
The <code>with</code> statement evaluates an expression and if that
expression is an object it is added to the <dfn>scope chain</dfn> of the current
execution context (in front of the Activation/Variable object). The
<code>with</code> statement then executes another statement (that may
itself be a block statement) and then restores the execution context's
<dfn>scope chain</dfn>to what it was before.
</p>

<p>
A function declaration could not be affected by a <code>with</code>
statement as they result in the creation of function objects during
variable instantiation, but a function expression can be evaluated
inside a <code>with</code> statement:-
</p>

<pre>
<span class="commentJS">/* create a global variable - y - that refers to an object:- */</span>
var y = {x:5}; <span class="commentJS">// object literal with an - x - property</span>
function exampleFuncWith(){
    var z;
    <span class="commentJS">/* Add the object referred to by the global variable - y - to the
       front of he scope chain:-
    */</span>
    with(y){
        <span class="commentJS">/* evaluate a function expression to create a function object
           and assign a reference to that function object to the local
           variable - z - :-
        */</span>
        z = function(){
            ... <span class="commentJS">// inner function expression body;</span>
        }
    }
    ... 
}

<span class="commentJS">/* execute the - exampleFuncWith - function:- */</span>
exampleFuncWith();
</pre>

<p>
When the <code>exampleFuncWith</code> function is called the resulting
execution context has a <dfn>scope chain</dfn> consisting of its Activation object
followed by the global object. The execution of the <code>with</code>
statement adds the object referred to by the global variable
<code>y</code> to the front of that <dfn>scope chain</dfn> during the evaluation
of the function expression. The function object created by the
evaluation of the function expression is assigned a
<code>[[scope]]</code> property that corresponds with the <dfn>scope</dfn> of the
execution context in which it is created. A <dfn>scope chain</dfn> consisting of
object <code>y</code> followed by the Activation object from the
execution context of the outer function call, followed by the global
object.
</p>

<p>
When the block statement associated with the <code>with</code>
statement terminates the <dfn>scope</dfn> of the execution context is restored
(the <code>y</code> object is removed), but the function object has
been created at that point and its <code>[[scope]]</code> property
assigned a reference to a <dfn>scope chain</dfn> with the <code>y</code> object
at its head.
</p>

<h3><a name="clIdRes" id="clIdRes">Identifier Resolution</a></h3>

<p>
Identifiers are resolved against the scope chain. ECMA 262 categorises
<code>this</code> as a keyword rather than an identifier, which is not
unreasonable as it is always resolved dependent on the
<code>this</code> value in the execution context in which it is used,
without reference to the scope chain.
</p>

<p>
Identifier resolution starts with the first object in the scope chain.
It is checked to see if it has a property with a name that corresponds
with the identifier. Because the <dfn>scope chain</dfn> is a chain of objects
this checking encompasses the prototype chain of that object (if it
has one). If no corresponding value can be found on the first object
in the <dfn>scope chain</dfn> the search progresses to the next object. And so on until
one of the objects in the chain (or one of its prototypes) has a
property with a name that corresponds with the identifier or the scope
chain is exhausted.
</p>

<p>
The operation on the identifier happens in the same way as the use of
property accessors on objects described above. The object identified
in the <dfn>scope chain</dfn> as having the corresponding property takes the
place of the object in the property accessor and the identifier acts
as a property name for that object. The global object is always at the
end of the scope chain.
</p>

<p>
As execution contexts associated with function calls will have the
Activation/Variable object at the front of the chain, identifiers used
in function bodies are effectively first checked to see whether they
correspond with formal parameters, inner function declaration names or
local variables. Those would be resolved as named properties of the
Activation/Variable object.
</p>

<h2><a name="clClose" id="clClose">Closures</a></h2>

<h3><a name="clAtGb" id="clAtGb">Automatic Garbage Collection</a></h3>

<p>
ECMAScript uses automatic garbage collection. The specification
does not define the details, leaving that to the implementers to sort
out, and some implementations are known to give a very low priority to
their garbage collection operations. But the general idea is that if an
object becomes un-referable (by having no remaining references to it
left accessible to executing code) it becomes available for garbage
collection and will at some future point be destroyed and any resources
it is consuming freed and returned to the system for re-use.
</p>

<p>
This would normally be the case upon exiting an execution context. The
<dfn>scope chain</dfn> structure, the Activation/Variable object and any objects
created within the execution context, including function objects, would
no longer be accessible and so would become available for garbage
collection.
</p>

<h3><a name="clFrmC" id="clFrmC">Forming Closures</a></h3>

<p>
A closure is formed by returning a function object that was created
within an execution context of a function call from that function call
and assigning a reference to that inner function to a property of another
object. Or by directly assigning a reference to such a function object
to, for example, a global variable, a property of a globally accessible
object or an object passed by reference as an argument to the outer 
function call. e.g:-
</p>

<pre>
function exampleClosureForm(arg1, arg2){
    var localVar = 8;
    function exampleReturned(innerArg){
        return ((arg1 + arg2)/(innerArg + localVar));
    }
    <span class="commentJS">/* return a reference to the inner function defined as -
       exampleReturned -:-
    */</span>
    return exampleReturned;
}

var globalVar = exampleClosureForm(2, 4);
</pre>

<p>
Now the function object created within the execution context of the
call to <code>exampleClosureForm</code> cannot be garbage collected
because it is referred to by a global variable and is still accessible,
it can even be executed with <code>globalVar(n)</code>.
</p>

<p>
But something a little more complicated has happened because the
function object now referred to by <code>globalVar</code> was created
with a <code>[[scope]]</code> property referring to a scope chain
containing the Activation/Variable object belonging to the execution
context in which it was created (and the global object). Now the
Activation/Variable object cannot be garbage collected either as the
execution of the function object referred to by <code>globalVar</code>
will need to add the whole <dfn>scope chain</dfn> from its <code>[[scope]]</code>
property to the <dfn>scope</dfn> of the execution context created for each call to
it.
</p>

<p>
A closure is formed. The inner function object has the free variables
and the Activation/Variable object on the function's <dfn>scope chain</dfn> is
the environment that binds them.
</p>

<p>
The Activation/Variable object is trapped by being
referred to in the <dfn>scope chain</dfn> assigned to the internal
<code>[[scope]]</code> property of the function object now referred to
by the <code>globalVar</code> variable. The Activation/Variable object
is preserved along with its state; the values of its properties. Scope
resolution in the execution context of calls to the inner function will
resolve identifiers that correspond with named properties of that
Activation/Variable object as properties of that object. The value of
those properties can still be read and set even though the execution
context for which it was created has exited.
</p>

<p>
In the example above that Activation/Variable object has a state that
represents the values of formal parameters, inner function definitions
and local variables, at the time when the outer function returned
(exited its execution context). The <code>arg1</code> property has the
value <code>2</code>,the <code>arg2</code> property the value
<code>4</code>, <code>localVar</code> the value <code>8</code> and an
<code>exampleReturned</code> property that is a reference to the inner
function object that was returned form the outer function. (We will be
referring to this Activation/Variable object as "ActOuter1" in later
discussion, for convenience.)
</p>

<p>
If the <code>exampleClosureForm</code> function was called again as:-
</p>

<pre>
var secondGlobalVar = exampleClosureForm(12, 3);
</pre>

<p>
- a new execution context would be created, along with a new Activation
object. And a new function object would be returned, with its own
distinct <code>[[scope]]</code> property referring to a scope chain
containing the Activation object form this second execution context,
with <code>arg1</code> being <code>12</code> and <code>arg2</code>
being <code>3</code>. (We will be referring to this Activation/Variable
object as &quot;ActOuter2&quot; in later discussion, for convenience.)
</p>

<p>
A second and distinct closure has been formed by the second execution
of <code>exampleClosureForm</code>.
</p>

<p>
The two function objects created by the execution of
<code>exampleClosureForm</code> to which references have been assigned
to the global variable <code>globalVar</code> and
<code>secondGlobalVar</code> respectively, return the expression
<code>((arg1 + arg2)/(innerArg + localVar))</code>. Which applies
various operators to four identifiers. How these identifiers are
resolved is critical to the use and value of closures.
</p>

<p>
Consider the execution of the function object referred to by
<code>globalVar</code>, as <code>globalVar(2)</code>. A new execution
context is created and an Activation object (we will call it
&quot;ActInner1&quot;), which is added to the head of the scope chain
referred to the <code>[[scope]]</code> property of the executed
function object. ActInner1 is given a property named
<code>innerArg</code>, after its formal parameter and the argument
value <code>2</code> assigned to it. The <dfn>scope chain</dfn> for this new
execution context is: <span class="scopeCh">ActInner1-&gt;</span>
<span class="scopeCh">ActOuter1-&gt;</span>
<span class="scopeCh">global object</span>.
</p>

<p>
Identifier resolution is done against the <dfn>scope chain</dfn> so in order
to return the value of the expression
<code>((arg1 + arg2)/(innerArg + localVar))</code> the values of the
identifiers will be determined by looking for properties, with names
corresponding with the identifiers, on each object in the scope chain
in turn.
</p>

<p>
The first object in the chain is ActInner1 and it has a property named
<code>innerArg</code> with the value <code>2</code>. All of the other
3 identifiers correspond with named properties of ActOuter1;
<code>arg1</code> is <code>2</code>, <code>arg2</code> is 
<code>4</code> and <code>localVar</code> is <code>8</code>. The
function call returns <code>((2 + 4)/(2 + 8))</code>.
</p>

<p>
Compare that with the execution of the otherwise identical function
object referred to by <code>secondGlobalVar</code>, as
<code>secondGlobalVar(5)</code>. Calling the Activation object for
this new execution context &quot;ActInner2&quot;, the scope chain
becomes: <span class="scopeCh">ActInner2-&gt;</span>
<span class="scopeCh">ActOuter2-&gt;</span>
<span class="scopeCh">global object</span>. ActInner2 returns
<code>innerArg</code> as <code>5</code> and ActOuter2 returns
<code>arg1</code>, <code>arg2</code> and <code>localVar</code> as
<code>12</code>, <code>3</code> and <code>8</code> respectively. The
value returned is <code>((12 + 3)/(5 + 8))</code>.
</p>

<p>
Execute <code>secondGlobalVar</code> again and a new Activation object
will appear at the front of the <dfn>scope chain</dfn> but ActOuter2 will still
be next object in the chain and the value of its named properties will
again be used in the resolution of the identifiers <code>arg1</code>,
<code>arg2</code> and <code>localVar</code>.
</p>

<p>
This is how ECMAScript inner functions gain, and maintain, access to the formal
parameters, declared inner functions and local variables of the
execution context in which they were created. And it is how the
forming of a closure allows such a function object to keep referring
to those values, reading and writing to them, for as long as it
continues to exist. The Activation/Variable object from the execution
context in which the inner function was created remains on the scope
chain referred to by the function object's <code>[[scope]]</code>
property, until all references to the inner function are freed and
the function object is made available for garbage collection (along
with any now unneeded objects on its scope chain).
</p>

<p>
Inner function may themselves have inner functions, and the inner
functions returned from the execution of functions to form closures
may themselves return inner functions and form closures of their own.
With each nesting the <dfn>scope chain</dfn> gains extra Activation objects
originating with the execution contexts in which the inner function
objects were created. The ECMAScript specification requires a scope
chain to be finite, but imposes no limits on their length.
Implementations probably do impose some practical limitation but no
specific magnitude has yet been reported. The potential for nesting
inner functions seems so far to have exceeded anyone's desire to
code them.
</p>

<h2><a name="clClDo" id="clClDo">What can be done with Closures?</a></h2>

<p>
Strangely the answer to that appears to be anything and everything.
I am told that closures enable ECMAScript to emulate anything, so the
limitation is the ability to conceive and implement the emulation. That
is a bit esoteric and it is probably better to start with something a
little more practical.
</p>


<h3><a name="clSto" id="clSto">Example 1: setTimeout with Function References</a></h3>


<p>
A common use for a closure is to provide parameters for the execution
of a function prior to the execution of that function. For example,
when a function is to be provided as the first argument to the
<code>setTimout</code> function that is common in web browser
environments.
</p>

<p>
<code>setTimeout</code> schedules the execution of a function (or a
string of javascript source code, but not in this context), provided as
its first argument, after an interval expressed in milliseconds (as its
second argument). If a piece of code wants to use
<code>setTimeout</code> it calls the <code>setTimeout</code> function
and passes a reference to a function object as the first argument and
the millisecond interval as the second, but a reference to a function
object cannot provide parameters for the scheduled execution of that
function.
</p>

<p>
However, code could call another function that returned a reference to
an inner function object, with that inner function object being passed
by reference to the <code>setTimeout</code> function. The parameters to
be used for the execution of the inner function are passed with the
call to the function that returns it. <code>setTimout</code> executes
the inner function without passing arguments but that inner function
can still access the parameters provided by the call to the outer
function that returned it:-
</p>

<pre>
function callLater(paramA, paramB, paramC){
    <span class="commentJS">/* Return a reference to an anonymous inner function created
       with a function expression:-
    */</span>
    return (function(){
        <span class="commentJS">/* This inner function is to be executed with - setTimeout
           - and when it is executed it can read, and act upon, the
           parameters passed to the outer function:-
        */</span>
        paramA[paramB] = paramC;
    });
}

...

<span class="commentJS">/* Call the function that will return a reference to the inner function
   object created in its execution context. Passing the parameters that
   the inner function will use when it is eventually executed as
   arguments to the outer function. The returned reference to the inner
   function object is assigned to a local variable:-
*/</span>
var functRef = callLater(elStyle, "display", "none");
<span class="commentJS">/* Call the setTimeout function, passing the reference to the inner
   function assigned to the - functRef - variable as the first argument:-
*/</span>
hideMenu=setTimeout(functRef, 500);
</pre>

<h3><a name="clObjI" id="clObjI">Example 2: Associating Functions with Object Instance Methods</a></h3>

<p>
There are many other circumstances when a reference to a function
object is assigned so that it would be executed at some future time
where it is useful to provide parameters for the execution of that
function that would not be easily available at the time of execution
but cannot be known until the moment of assignment.
</p>

<p>
One example might be a javascript object that is designed to
encapsulate the interactions with a particular DOM element. It has
<code>doOnClick</code>, <code>doMouseOver</code> and
<code>doMouseOut</code> methods and wants to execute those methods
when the corresponding events are triggered on the DOM element, but
there may be any number of instances of the javascript object created
associated with different DOM elements and the individual object
instances do not know how they will be employed by the code that
instantiated them. The object instances do not know how to reference
themselves globally because they do not know which global variables
(if any) will be assigned references to their instances.
</p>

<p>
So the problem is to execute an event handling function that has an
association with a particular instance of the javascript object, and
knows which method of that object to call.
</p>

<p>
The following example uses a small generalised closure based function
that associates object instances with element event handlers.
Arranging that the execution of the event handler calls the specified
method of the object instance, passing the event object and a reference
to the associated element on to the object method and returning the
method's return value.
</p>

<pre>
<span class="commentJS">/* A general function that associates an object instance with an event
   handler. The returned inner function is used as the event handler.
   The object instance is passed as the - obj - parameter and the name
   of the method that is to be called on that object is passed as the -
   methodName - (string) parameter.
*/</span>
function associateObjWithEvent(obj, methodName){
    <span class="commentJS">/* The returned inner function is intended to act as an event
       handler for a DOM element:-
    */</span>
    return (function(e){
        <span class="commentJS">/* The event object that will have been parsed as the - e -
           parameter on DOM standard browsers is normalised to the IE
           event object if it has not been passed as an argument to the
           event handling inner function:-
        */</span>
        e = e||window.event;
        <span class="commentJS">/* The event handler calls a method of the object - obj - with
           the name held in the string - methodName - passing the now
           normalised event object and a reference to the element to
           which the event handler has been assigned using the - this -
           (which works because the inner function is executed as a
           method of that element because it has been assigned as an
           event handler):-
        */</span>
        return obj[methodName](e, this);
    });
}

<span class="commentJS">/* This constructor function creates objects that associates themselves
   with DOM elements whose IDs are passed to the constructor as a
   string. The object instances want to arrange than when the
   corresponding element triggers onclick, onmouseover and onmouseout
   events corresponding methods are called on their object instance.
*/</span>
function DhtmlObject(elementId){
    <span class="commentJS">/* A function is called that retrieves a reference to the DOM
       element (or null if it cannot be found) with the ID of the
       required element passed as its argument. The returned value
       is assigned to the local variable - el -:-
    */</span>
    var el = getElementWithId(elementId);
    <span class="commentJS">/* The value of - el - is internally type-converted to boolean for
       the - if - statement so that if it refers to an object the
       result will be true, and if it is null the result false. So that
       the following block is only executed if the - el - variable
       refers to a DOM element:-
    */</span>
    if(el){
        <span class="commentJS">/* To assign a function as the element's event handler this
           object calls the - associateObjWithEvent - function
           specifying itself (with the - this - keyword) as the object
           on which a method is to be called and providing the name of
           the method that is to be called. The - associateObjWithEvent
           - function will return a reference to an inner function that
           is assigned to the event handler of the DOM element. That
           inner function will call the required method on the
           javascript object when it is executed in response to
           events:-
        */</span>
        el.onclick = associateObjWithEvent(this, "doOnClick");
        el.onmouseover = associateObjWithEvent(this, "doMouseOver");
        el.onmouseout = associateObjWithEvent(this, "doMouseOut");
        ...
    }
}
DhtmlObject.prototype.doOnClick = function(event, element){
    ... <span class="commentJS">// doOnClick method body</span>.
}
DhtmlObject.prototype.doMouseOver = function(event, element){
    ... <span class="commentJS">// doMouseOver method body.</span>
}
DhtmlObject.prototype.doMouseOut = function(event, element){
    ... <span class="commentJS">// doMouseOut method body.</span>
}
</pre>

<p>
And so any instances of the <code>DhtmlObject</code> can associate themselves
with the DOM element that they are interested in without any need
to know anything about how they are being employed by other code,
impacting on the global namespace or risking clashes with other
instances of the <code>DhtmlObject</code>.
</p>

<h3><a name="clEncap" id="clEncap">Example 3: Encapsulating Related Functionality</a></h3>

<p>
Closures can be used to create additional scopes that can be used to
group interrelated and dependent code in a way that minimises the risk
of accidental interaction. Suppose a function is to build a string and
to avoid the repeated concatenation operations (and the creation of
numerous intermediate strings) the desire is to use an array to store
the parts of the string in sequence and then output the results using
the <code>Array.prototype.join</code> method (with an empty string as its argument).
The array is going to act as a buffer for the output, but defining it
locally to the function will result in its re-creation on each
execution of the function, which may not be necessary if the only
variable content of that array will be re-assigned on each function
call.
</p>

<p>
One approach might make the array a global variable so that it can be
re-used without being re-created. But the consequences of that will be
that, in addition to the global variable that refers to the function
that will use the buffer array, there will be a second global property
that refers to the array itself. The effect is to render the code less
manageable, as, if it is to be used elsewhere, its author has to remember
to include both the function definition and the array definition. It
also makes the code less easy to integrate with other code because
instead of just ensuring that the function name is unique within the
global namespace it is necessary to ensure that the Array on which it
is dependent is using a name that is unique within the global
namespace.
</p>

<p>
A Closure allows the buffer array to be associated (and neatly
packaged) with the function that is dependent upon it and
simultaneously keep the property name to which the buffer array as
assigned out of the global namespace and free of the risk of name
conflicts and accidental interactions.
</p>

<p>
The trick here is to create one additional execution context by
executing a function expression in-line and have that function
expression return an inner function that will be the function that is
used by external code. The buffer array is then defined as a local
variable of the function expression that is executed in-line. That only
happens once so the Array is only created once, but is available to
the function that depends on it for repeated use.
</p>

<p>
The following code creates a function that will return a string of
<span class="initialism" title="HyperText Mark-up Language"><abbr title="HyperText Mark-up Language">HTML</abbr></span>, much of which is constant, but those constant character
sequences need to be interspersed with variable information provided
as parameter to the function call.
</p>

<p>
A reference to an inner function object is returned from the in-line
execution of a function expression and assigned to a global variable
so that it can be called as a global function. The buffer array is
defined as a local variable in the outer function expression. It is
not exposed in the global namespace and does not need to be re-created
whenever the function that uses it is called.
</p> 

<pre>
<span class="commentJS">/* A global variable - getImgInPositionedDivHtml - is declared and
   assigned the value of an inner function expression returned from
   a one-time call to an outer function expression.

   That inner function returns a string of <span class="initialism" title="HyperText Mark-up Language"><abbr title="HyperText Mark-up Language">HTML</abbr></span> that represents an
   absolutely positioned DIV wrapped round an IMG element, such that
   all of the variable attribute values are provided as parameters
   to the function call:-
*/</span>
var getImgInPositionedDivHtml = (function(){
    <span class="commentJS">/* The - buffAr - Array is assigned to a local variable of the
       outer function expression. It is only created once and that one
       instance of the array is available to the inner function so that
       it can be used on each execution of that inner function.

       Empty strings are used as placeholders for the date that is to
       be inserted into the Array by the inner function:-
    */</span>
    var buffAr = [
        '&lt;div id="',
        '',   <span class="commentJS">//index 1, DIV ID attribute</span>
        '" style="position:absolute;top:',
        '',   <span class="commentJS">//index 3, DIV top position</span>
        'px;left:',
        '',   <span class="commentJS">//index 5, DIV left position</span>
        'px;width:',
        '',   <span class="commentJS">//index 7, DIV width</span>
        'px;height:',
        '',   <span class="commentJS">//index 9, DIV height</span>
        'px;overflow:hidden;\"&gt;&lt;img src=\"',
        '',   <span class="commentJS">//index 11, IMG URL</span>
        '\" width=\"',
        '',   <span class="commentJS">//index 13, IMG width</span>
        '\" height=\"',
        '',   <span class="commentJS">//index 15, IMG height</span>
        '\" alt=\"',
        '',   <span class="commentJS">//index 17, IMG alt text</span>
        '\"&gt;&lt;\/div&gt;'
    ];
    <span class="commentJS">/* Return the inner function object that is the result of the
       evaluation of a function expression. It is this inner function
       object that will be executed on each call to -
       getImgInPositionedDivHtml( ... ) -:-
    */</span>
    return (function(url, id, width, height, top, left, altText){
        <span class="commentJS">/* Assign the various parameters to the corresponding
           locations in the buffer array:-
        */</span>
        buffAr[1] = id;
        buffAr[3] = top;
        buffAr[5] = left;
        buffAr[13] = (buffAr[7] = width);
        buffAr[15] = (buffAr[9] = height);
        buffAr[11] = url;
        buffAr[17] = altText;
        <span class="commentJS">/* Return the string created by joining each element in the
           array using an empty string (which is the same as just
           joining the elements together):-
        */</span>
        return buffAr.join('');
    }); <span class="commentJS">//:End of inner function expression.</span>
})();
<span class="commentJS">/*^^- :The inline execution of the outer function expression. */</span>
</pre>

<p>
If one function was dependent on one (or several) other functions, but
those other functions were not expected to be directly employed by any
other code, then the same technique could be used to group those
functions with the one that was to be publicly exposed. Making a
complex multi-function process into an easily portable and encapsulated
unit of code.
</p>

<h3><a name="clOtE" id="clOtE">Other Examples</a></h3>

<p>
Probably one of the best known applications of closures is
<a href="http://www.crockford.com/javascript/private.html">Douglas
Crockford's technique for the emulation of private instance variables
in ECMAScript objects</a>. Which can be extended to all sorts of
structures of <dfn>scope</dfn> contained nested accessibility/visibility, including
<a href="http://myweb.tiscali.co.uk/cornford/js_info/private_static.html">
the emulation of private static members for ECMAScript objects</a>.
</p>

<p>
The possible application of closures are endless, understanding how
they work is probably the best guide to realising how they can be
used.
</p>

<h2 id="clAc">Accidental Closures</h2>

<p>
Rendering any inner function accessible outside of the body of the
function in which it was created will form a closure. That makes
closures very easy to create and one of the consequences is that
javascript authors who do not appreciate closures as a language feature
can observe the use of inner functions for various tasks and employ
inner functions, with no apparent consequences, not realising that
closures are being created or what the implications of doing that are.
</p>

<p>
Accidentally creating closures can have harmful side effects as the
following section on the IE memory leak problem describes, but they can
also impact of the efficiency of code. It is not the closures
themselves, indeed carefully used they can contribute significantly
towards the creation of efficient code. It is the use of inner
functions that can impact on efficiency.
</p>

<p>
A common situation is where inner functions are used is as event
handlers for DOM elements. For example the following code might be used
to add an onclick handler to a link element:-
</p>

<pre>
<span class="commentJS">/* Define the global variable that is to have its value added to the
   - href - of a link as a query string by the following function:-
*/</span>
var quantaty = 5;
<span class="commentJS">/* When a link passed to this function (as the argument to the function
   call - linkRef -) an onclick event handler is added to the link that
   will add the value of a global variable - quantaty - to the - href -
   of that link as a query string, then return true so that the link
   will navigate to the resource specified by the - href - which will
   by then include the assigned query string:-
*/</span>
function addGlobalQueryOnClick(linkRef){
    <span class="commentJS">/* If the - linkRef - parameter can be type converted to true
       (which it will if it refers to an object):-
    */</span>
    if(linkRef){
        <span class="commentJS">/* Evaluate a function expression and assign a reference to the
           function object that is created by the evaluation of the
           function expression to the onclick handler of the link
           element:-
        */</span>
        linkRef.onclick = function(){
            <span class="commentJS">/* This inner function expression adds the query string to
               the - href - of the element to which it is attached as
               an event handler:-
            */</span>
            this.href += ('?quantaty='+escape(quantaty));
            return true;
        };
    }
}
</pre>

<p>
Whenever the <code>addGlobalQueryOnClick</code> function is called a
new inner function is created (and a closure formed by its assignment).
From the efficiency point of view that would not be significant if the
<code>addGlobalQueryOnClick</code> function was only called once or
twice, but if the function was heavily employed many distinct function
objects would be created (one for each evaluation of the inner function
expression).
</p>

<p>
The above code is not taking advantage of the fact that inner functions
are becoming accessible outside of the function in which they are being
created (or the resulting closures). As a result exactly the same effect
could be achieved by defining the function that is to be used as the
event handler separately and then assigning a reference to that
function to the event handling property. Only one function object would
be created and all of the elements that use that event handler would
share a reference to that one function:-
</p>

<pre>
<span class="commentJS">/* Define the global variable that is to have its value added to the
   - href - of a link as a query string by the following function:-
*/</span>
var quantaty = 5;

<span class="commentJS">/* When a link passed to this function (as the argument to the function
   call - linkRef -) an onclick event handler is added to the link that
   will add the value of a global variable - quantaty - to the - href -
   of that link as a query string, then return true so that the link
   will navigate to the resource specified by the - href - which will
   by then include the assigned query string:-
*/</span>
function addGlobalQueryOnClick(linkRef){
    <span class="commentJS">/* If the - linkRef - parameter can be type converted to true
       (which it will if it refers to an object):-
    */</span>
    if(linkRef){
        <span class="commentJS">/* Assign a reference to a global function to the event
           handling property of the link so that it becomes the
           element's event handler:-
        */</span>
        linkRef.onclick = forAddQueryOnClick;
    }
}
<span class="commentJS">/* A global function declaration for a function that is intended to act
   as an event handler for a link element, adding the value of a global
   variable to the - href - of an element as an event handler:-
*/</span>
function forAddQueryOnClick(){
    this.href += ('?quantaty='+escape(quantaty));
    return true;
}
</pre>

<p>
As the inner function in the first version is not being used to exploit
the closures produced by its use, it would be more efficient not to use
an inner function, and thus not repeat the process of creating many
essentially identical function objects.
</p>

<p>
A similar consideration applies to object constructor functions. It is
not uncommon to see code similar to the following skeleton constructor:-
</p>


<pre>
function ExampleConst(param){
    <span class="commentJS">/* Create methods of the object by evaluating function expressions
       and assigning references to the resulting function objects
       to the properties of the object being created:-
    */</span>
    this.method1 = function(){
        ... <span class="commentJS">// method body.</span>
    };
    this.method2 = function(){
        ... <span class="commentJS">// method body.</span>
    };
    this.method3 = function(){
        ... <span class="commentJS">// method body.</span>
    };
    <span class="commentJS">/* Assign the constructor's parameter to a property of the object:-
    */</span>
    this.publicProp = param;
}
</pre>

<p>
Each time the constructor is used to create an object, with
<code>new ExampleConst(n)</code>, a new set of function objects are
created to act as its methods. So the more object instances that are
created the more function objects are created to go with them.
</p>

<p>
Douglas Crockford's technique for emulating private members on
javascript objects exploits the closure resulting form assigning
references to inner function objects to the public properties of a
constructed object from within its constructor. But if the methods of
an object are not taking advantage of the closure that they will form
within the constructor the creation of multiple function objects for
each object instantiation will make the instantiation process slower
and more resources will be consumed to accommodate the extra function
objects created.
</p>

<p>
In that case it would be more efficient to create the function object
once and assign references to them to the corresponding properties of
the constructor's <code>prototype</code> so they may be shared by all
of the objects created with that constructor:-
</p>

<pre>
function ExampleConst(param){
    <span class="commentJS">/* Assign the constructor's parameter to a property of the object:-
    */</span>
    this.publicProp = param;
}
<span class="commentJS">/* Create methods for the objects by evaluating function expressions
   and assigning references to the resulting function objects to the
   properties of the constructor's prototype:-
*/</span>
ExampleConst.prototype.method1 = function(){
    ... <span class="commentJS">// method body.</span>
};
ExampleConst.prototype.method2 = function(){
    ... <span class="commentJS">// method body.</span>
};
ExampleConst.prototype.method3 = function(){
    ... <span class="commentJS">// method body.</span>
};

</pre>
<h2><a  name="clMem" id="clMem">The Internet Explorer Memory Leak Problem</a></h2>

<p>
The Internet Explorer web browser (verified on versions 4 to 6 (6 is
current at the time of writing)) has a fault in its garbage collection
system that prevents it from garbage collecting ECMAScript and some
host objects if those host objects  form part of a &quot;circular&quot;
reference. The host objects in question are any DOM Nodes (including
the document object and its descendants) and ActiveX objects. If a
circular reference is formed including one or more of them, then
none of the objects involved will be freed until the browser is closed
down, and the memory that they consume will be unavailable to the
system until that happens.
</p>

<p>
A circular reference is when two or more objects refer to each other in
a way that can be followed and lead back to the starting point. Such
as object 1 has a property that refers to object 2, object 2 has a
property that refers to object 3 and object 3 has a property that
refers back to object 1. With pure ECMAScript objects as soon as no
other objects refer to any of objects 1, 2 or 3 the fact that they only
refer to each other is recognised and they are made available for
garbage collection. But on Internet Explorer, if any of those objects
happen to be a DOM Node or ActiveX object, the garbage collection
cannot see that the circular relationship between them is isolated
from the rest of the system and free them. Instead they all stay in
memory until the browser is closed.
</p>

<p>
Closures are extremely good at forming circular references. If a
function object that forms a closure is assigned as, for example, and
event handler on a DOM Node, and a reference to that Node is assigned
to one of the Activation/Variable objects in its <dfn>scope chain</dfn> then a
circular reference exists.
<span class="scopeCh">DOM_Node.onevent -&gt;</span>
<span class="scopeCh">function_object.[[scope]] -&gt;</span>
<span class="scopeCh">scope_chain -&gt;</span>
<span class="scopeCh">Activation_object.nodeRef -&gt;</span>
<span class="scopeCh">DOM_Node</span>.
It is very easy to do, and a bit of browsing around a site that forms
such a reference in a piece of code common to each page can consume
most of the systems memory (possibly all).
</p>

<p>
Care can be taken to avoid forming circular references and remedial
action can be taken when they cannot otherwise be avoided, such as
using IE's onunload event to null event handling function
references. Recognising the problem and understanding closures
(and their mechanism) is the key to avoiding this problem with IE.
</p>

<p id="rToc">
<a href="faq_notes.html#toc">comp.lang.javascript FAQ notes T.O.C.</a>
</p>

<ul style="list-style-type:none;margin-top:2.5em;">
	<li>Written by <span class="person">Richard Cornford</span>. March 2004.</li>
	<li>With corrections and suggestions by:-<br>
		<ul style="list-style-type:none;">
			<li><span class="person">Martin Honnen</span>.</li>
			<li><span class="person">Yann-Erwan Perio (Yep)</span>.</li>
			<li><span class="person">Lasse Reichstein Nielsen</span>. (<a href="#clDefN">definition of closure</a>)</li>
			<li><span class="person">Mike Scirocco</span>.</li>
			<li><span class="person">Dr John Stockton</span>.</li>
			<li><span class="person">Garrett Smith</span>.</li>
		</ul>
	</li>
</ul>
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