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---
title: Function
slug: Web/JavaScript/Reference/Global_Objects/Function
tags:
- Constructor
- Function
- JavaScript
- NeedsTranslation
- TopicStub
translation_of: Web/JavaScript/Reference/Global_Objects/Function
---
<div>{{JSRef}}</div>
<p>The <strong><code>Function</code> constructor</strong> creates a new <code>Function</code> object. Calling the constructor directly can create functions dynamically, but suffers from security and similar (but far less significant) performance issues similar to {{jsxref("eval")}}. However, unlike eval, the Function constructor allows executing code in the global scope, prompting better programming habits and allower for more efficient code minification.</p>
<div>{{EmbedInteractiveExample("pages/js/function-constructor.html")}}</div>
<p>Every JavaScript function is actually a <code>Function</code> object. This can be seen with the code <code>(function(){}).constructor === Function</code> which returns true.</p>
<h2 id="Syntax">Syntax</h2>
<pre class="syntaxbox"><code>new Function ([<var>arg1</var>[, <var>arg2</var>[, ...<var>argN</var>]],] <var>functionBody</var>)</code></pre>
<h3 id="Parameters">Parameters</h3>
<dl>
<dt><code>arg1, arg2, ... arg<em>N</em></code></dt>
<dd>Names to be used by the function as formal argument names. Each must be a string that corresponds to a valid JavaScript identifier or a list of such strings separated with a comma; for example "<code>x</code>", "<code>theValue</code>", or "<code>a,b</code>".</dd>
<dt><code>functionBody</code></dt>
<dd>A string containing the JavaScript statements comprising the function definition.</dd>
</dl>
<h2 id="Description">Description</h2>
<p><code>Function</code> objects created with the <code>Function</code> constructor are parsed when the function is created. This is less efficient than declaring a function with a <a href="/en-US/docs/Web/JavaScript/Reference/Operators/function">function expression</a> or <a href="/en-US/docs/Web/JavaScript/Reference/Statements/function">function statement</a> and calling it within your code because such functions are parsed with the rest of the code.</p>
<p>All arguments passed to the function are treated as the names of the identifiers of the parameters in the function to be created, in the order in which they are passed.</p>
<p>Invoking the <code>Function</code> constructor as a function (without using the <code>new</code> operator) has the same effect as invoking it as a constructor. However, getting rid of the new operator allows for a smaller minified code size (4 bytes smaller), so it is best to not use <code>new</code> with <code>Function</code>.</p>
<h2 id="Properties_and_Methods_of_Function">Properties and Methods of <code>Function</code></h2>
<p>The global <code>Function</code> object has no methods or properties of its own. However, since it is a function itself, it does inherit some methods and properties through the prototype chain from {{jsxref("Function.prototype")}}.</p>
<h2 id="Function_prototype_object"><code>Function</code> prototype object</h2>
<h3 id="Properties">Properties</h3>
<div>{{page('/en-US/docs/JavaScript/Reference/Global_Objects/Function/prototype', 'Properties')}}</div>
<h3 id="Methods">Methods</h3>
<div>{{page('/en-US/docs/JavaScript/Reference/Global_Objects/Function/prototype', 'Methods')}}</div>
<h2 id="Function_instances"><code>Function</code> instances</h2>
<p><code>Function</code> instances inherit methods and properties from {{jsxref("Function.prototype")}}. As with all constructors, you can change the constructor's prototype object to make changes to all <code>Function</code> instances.</p>
<h2 id="Examples">Examples</h2>
<h3 id="Specifying_arguments_with_the_Function_constructor">Specifying arguments with the <code>Function</code> constructor</h3>
<p>The following code creates a <code>Function</code> object that takes two arguments.</p>
<pre class="brush: js">// Example can be run directly in your JavaScript console
// Create a function that takes two arguments and returns the sum of those arguments
var adder = new Function('a', 'b', 'return a + b');
// Call the function
adder(2, 6);
// > 8
</pre>
<p>The arguments "<code>a</code>" and "<code>b</code>" are formal argument names that are used in the function body, "<code>return a + b</code>".</p>
<h3 id="Difference_between_Function_constructor_and_function_declaration">Difference between Function constructor and function declaration</h3>
<p>Functions created with the <code>Function</code> constructor do not create closures to their creation contexts; they always are created in the global scope. When running them, they will only be able to access their own local variables and global ones, not the ones from the scope in which the <code>Function</code> constructor was called. This is different from using {{jsxref("eval")}} with code for a function expression.</p>
<pre class="brush: js">var x = 10;
function createFunction1() {
var x = 20;
return new Function('return x;'); // this |x| refers global |x|
}
function createFunction2() {
var x = 20;
function f() {
return x; // this |x| refers local |x| above
}
return f;
}
var f1 = createFunction1();
console.log(f1()); // 10
var f2 = createFunction2();
console.log(f2()); // 20
</pre>
<p>The "proper" way to execute external code with <code>Function</code> (for maximum minifyability).</p>
<pre class="brush: js">function makeFunction(code){
return Function('"use strict";return ' + code)();
}
var add = makeFunction(
"" + function(a, b, c){ return a + b + c } // move this to a separate file in the production release
);
console.log( add(1, 2, 3) ); // will log six</pre>
<p>Please note that the above code by itself is completely impractical. You should never abuse Function like that. Instead, the above code is meant only to be a simplified example of something like a module loader where there is a base script, then there are hundreads of big optionally loaded modules. Then, instead of the user waiting while they all download, the clients computer only downloads modules as needed so the page loads super fast. Also, it is reccomended that when evaluating many functions, the functions are evaluated together in bulk instead of separatly.</p>
<pre class="brush: js">function bulkMakeFunctions(){
var str = "", i = 1, Len = arguments.length;
if (Len) {
str = arguments[0];
while (i !== Len) str += "," + arguments[i], ++i;
}
return Function('"use strict";return[' + str + ']')();
}
const [
add, sub, mul, div
] = bulkMakeFunctions(
"function(a,b){return a+b}","function(a,b){return a-b}","function(a,b){return a*b}","function(a,b){return a/b}"
);
console.log(sub(add(mul(4,3), div(225,5)), 7))
</pre>
<h2 id="Specifications">Specifications</h2>
<table class="standard-table">
<tbody>
<tr>
<th scope="col">Specification</th>
<th scope="col">Status</th>
<th scope="col">Comment</th>
</tr>
<tr>
<td>{{SpecName('ES1')}}</td>
<td>{{Spec2('ES1')}}</td>
<td>Initial definition. Implemented in JavaScript 1.0.</td>
</tr>
<tr>
<td>{{SpecName('ES5.1', '#sec-15.3', 'Function')}}</td>
<td>{{Spec2('ES5.1')}}</td>
<td> </td>
</tr>
<tr>
<td>{{SpecName('ES6', '#sec-function-objects', 'Function')}}</td>
<td>{{Spec2('ES6')}}</td>
<td> </td>
</tr>
<tr>
<td>{{SpecName('ESDraft', '#sec-function-objects', 'Function')}}</td>
<td>{{Spec2('ESDraft')}}</td>
<td> </td>
</tr>
</tbody>
</table>
<h2 id="Browser_compatibility">Browser compatibility</h2>
<div>
<p>{{Compat("javascript.builtins.Function")}}</p>
</div>
<h2 id="See_also">See also</h2>
<ul>
<li>{{jsxref("Functions", "Functions and function scope")}}</li>
<li>{{jsxref("Function")}}</li>
<li>{{jsxref("Statements/function", "function statement")}}</li>
<li>{{jsxref("Operators/function", "function expression")}}</li>
<li>{{jsxref("Statements/function*", "function* statement")}}</li>
<li>{{jsxref("Operators/function*", "function* expression")}}</li>
<li>{{jsxref("AsyncFunction")}}</li>
<li>{{jsxref("GeneratorFunction")}}</li>
</ul>
|
