duktape/website/guide/functionobjects.html

<h1 id="functionobjects">Function objects</h1>

<h2 id="ecmascript-function-properties">Properties of Ecmascript functions</h2>

<p>Duktape Function objects add a few properties to standard Ecmascript
properties.  The table below summarizes properties assigned to newly
created function instances (properties can of course be added or removed
afterwards):</p>

<table>
<thead>
<tr><th>Property name</th><th>Compatibility</th><th>Description</th></tr>
</thead>
<tbody>
<tr><td class="propname">length</td><td>standard</td><td>Function (nominal) argument count (if relevant).  Present for all Function objects, including bound functions.</td></tr>
<tr><td class="propname">prototype</td><td>standard</td><td>Prototype used for new objects when called as a constructor.  Present for most constructable Function objects, not copied to bound functions.</td></tr>
<tr><td class="propname">caller</td><td>standard</td><td>Accessor which throws an error.  Present for strict functions and bound functions.  Not copied to bound functions.  (If <code>DUK_OPT_NONSTD_FUNC_CALLER_PROPERTY</code> is given, non-strict functions will get a non-standard <code>caller</code> property.)</td></tr>
<tr><td class="propname">arguments</td><td>standard</td><td>Accessor which throws an error.  Present for strict functions and bound functions.  Not copied to bound functions.</td></tr>
<tr><td class="propname">name</td><td>Duktape</td><td>Function name, see below.  Copied to bound function from target function.</td></tr>
<tr><td class="propname">fileName</td><td>Duktape</td><td>Filename or context where function was declared (same name as in error tracebacks).  Copied to bound function from target function. </td></tr>
<tr><td class="propname">callee</td><td>n/a</td><td>Never assigned by default (listed here to clarify relationship to "caller" property).</td></tr>
</tbody>
</table>

<p>The <code>name</code> property is assigned to all functions and is also
the name used in tracebacks.  It is assigned as follows:</p>
<pre class="ecmascript-code">
function funcDecl() {
    /* Function declaration: 'name' is declaration name, here 'funcDecl'. */
}

var foo = function namedFunc() {
    /* Named function expression: 'name' is the name used in expression,
     * here 'namedFunc' (not 'foo').
     */
}

var bar = function () {
    /* Anonymous function expression: 'name' is the empty string. */
}
</pre>

<div class="note">
Several Ecmascript built-in functions have properties different from user
created Functions.
</div>

<h2 id="duktapec-function-properties">Properties of Duktape/C functions</h2>

<p>User-created Duktape/C functions (<code>duk_push_c_function()</code>) have
a different set of properties to reduce Function object memory footprint:</p>

<table>
<thead>
<tr><th>Property name</th><th>Compatibility</th><th>Description</th></tr>
</thead>
<tbody>
<tr><td class="propname">length</td><td>standard</td><td>
Function argument count, matches argument to <code>duk_push_c_function()</code>, 0 for varargs.
Non-writable and non-configurable.</td></tr>
</tbody>
</table>

<p>Note in particular that the standard <code>prototype</code>, <code>caller</code>,
and <code>arguments</code> properties are missing by default.  This is not strictly
compliant but is important to reduce function footprint.  User code can of course
assign these but is not required to do so.</p>

<h2 id="duktapec-lightfunc-properties">Properties of lightweight Duktape/C functions</h2>

<p>Lightweight functions have a set of non-configurable, non-writable virtual
properties listed below.  Like normal functions, they inherit further properties
from <code>Function.prototype</code>.</p>

<table>
<thead>
<tr><th>Property name</th><th>Compatibility</th><th>Description</th></tr>
</thead>
<tbody>
<tr><td class="propname">length</td><td>standard</td><td>Function (nominal) argument count.</td></tr>
<tr><td class="propname">name</td><td>Duktape</td><td>Function name: <code>"light_&lt;PTR&gt;_&lt;FLAGS&gt;"</code>.</td></tr>
</tbody>
</table>

<p>The <code>name</code> property is an automatically generated virtual
function name. &lt;PTR&gt; is a platform dependent dump of the Duktape/C
function pointer, and &lt;FLAGS&gt; is a raw hex dump of the 16-bit internal
control fields (the format is Duktape internal).  You shouldn't rely on a
specific format.  For example:</p>
<pre>
duk&gt; print(myLightFunc.name);
light_0805b94c_0511
</pre>

<p>As for ordinary functions, a lightfunc coerces to an implementation
dependent string.  You shouldn't rely on a specific format.  For example:</p>
<pre>
duk&gt; print(myLightFunc);
function light_0805b94c_0511() {/* light */}
</pre>