Function

The Function constructor creates a new Function object. Calling the constructor directly can create functions dynamically, but suffers from security and similar (but far less significant) performance issues to eval. However, unlike eval, the Function constructor creates functions which execute in the global scope only.

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Every JavaScript function is actually a Function object. This can be seen with the code (function(){}).constructor === Function which returns true.

Syntax

new Function([arg1 [, arg2 [, ...argN]] ,] functionBody)

Parameters

arg1, arg2, ... argN

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: "x", "theValue"—or "x,theValue".

functionBody

A string containing the JavaScript statements comprising the function definition.

Description

Function objects created with the Function constructor are parsed when the function is created. This is less efficient than declaring a function with a function expression or function statement and calling it within your code because such functions are parsed with the rest of the code.

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. Omitting an argument will result in the value of that parameter being undefined.

Invoking the Function constructor as a function (without using the new operator) has the same effect as invoking it as a constructor.

Properties and Methods of Function

The global Function 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 Function.prototype.

Function prototype object

Properties

Methods

Function instances

Function instances inherit methods and properties from Function.prototype. As with all constructors, you can change the constructor's prototype object to make changes to all Function instances.

Examples

Specifying arguments with the Function constructor

The following code creates a Function object that takes two arguments.

// Example can be run directly in your JavaScript console

// Create a function that takes two arguments, and returns the sum of those arguments
const adder = new Function('a', 'b', 'return a + b');

// Call the function
adder(2, 6);
// 8

The arguments "a" and "b" are formal argument names that are used in the function body, "return a + b".

Difference between Function constructor and function declaration

Functions created with the Function 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 Function constructor was created. This is different from using eval with code for a function expression.

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

While this code works in web browsers, f1() will produce a ReferenceError in Node.js, as x will not be found. This is because the top-level scope in Node is not the global scope, and x will be local to the module.

Specifications

Browser compatibility

See also

• Functions and function scope
• function statement
• function expression
• function* statement
• function* expression
• AsyncFunction
• GeneratorFunction