How to Use Function "Constructors"

Site:	Saylor Academy
Course:	PRDV402: Introduction to JavaScript II
Book:	How to Use Function "Constructors"

Printed by:	Guest user
Date:	Wednesday, May 14, 2025, 2:04 AM

Description

Functions can be created using a "function" statement or the Function() constructor. Dynamic functions are created when defining functions with the "new" operator and Function() constructor. Constructors not only create new objects but also specify the behaviors and properties of those objects.

Function
Constructor
Instance properties
Instance methods
Examples

Function

The Function object provides methods for functions. In JavaScript, every function is actually a Function object.

Source: Mozilla, https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function
This work is licensed under a Creative Commons Attribution-ShareAlike 2.5 License.

Constructor

Function(): 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 that execute in the global scope only.

Instance properties

These properties are defined on Function.prototype and shared by all Function instances.

Function.prototype.arguments Deprecated Non-standard: Represents the arguments passed to this function. For strict, arrow, async, and generator functions, accessing the arguments property throws a TypeError. Use the arguments object inside function closures instead.
Function.prototype.caller Deprecated Non-standard: Represents the function that invoked this function. For strict, arrow, async, and generator functions, accessing the caller property throws a TypeError.
Function.prototype.constructor: The constructor function that created the instance object. For Function instances, the initial value is the Function constructor.

These properties are own properties of each Function instance.

displayName Non-standard Optional: The display name of the function.
length: Specifies the number of arguments expected by the function.
name: The name of the function.
prototype: Used when the function is used as a constructor with the new operator. It will become the new object's prototype.

Instance methods

Function.prototype.apply(): Calls a function with a given this value and optional arguments provided as an array (or an array-like object).
Function.prototype.bind(): Creates a new function that, when called, has its this keyword set to a provided value, optionally with a given sequence of arguments preceding any provided when the new function is called.
Function.prototype.call(): Calls a function with a given this value and optional arguments.
Function.prototype.toString(): Returns a string representing the source code of the function. Overrides the Object.prototype.toString method.
Function.prototype[@@hasInstance](): Specifies the default procedure for determining if a constructor function recognizes an object as one of the constructor's instances. Called by the instanceof operator.

Examples

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.

// Create a global property with `var`
var x = 10;

function createFunction1() {
  const x = 20;
  return new Function("return x;"); // this `x` refers to global `x`
}

function createFunction2() {
  const x = 20;
  function f() {
    return x; // this `x` refers to the local `x` above
  }
  return f;
}

const f1 = createFunction1();
console.log(f1()); // 10
const 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.