JS: What's Constructor?

By Xah Lee. Date: . Last updated: .

What's a Constructor?

When a function is designed to be called with the keyword new, for example new function_name(…), we call the function constructor.

(There are some exceptions. For example, the constructor Symbol cannot be called with new [see JS: Symbol Tutorial])

JavaScript spec defines constructor thus:

function object that creates and initializes objects

NOTE The value of a constructor's prototype property is a prototype object that is used to implement inheritance and shared properties.

ECMAScript 2015 §Overview#sec-constructor

Constructor is one of the way to create a object. [see JS: Create Object]

Builtin Constructors

The most common use of new is to create a builtin object instances using builtin constructor, such as Date object.

const x = new Date();

Here's example of standard builtin constructors.

Common Builtin Construtors
ObjectLiteral Expression SyntaxConstructor SyntaxNotes
Object{…}new Object(…) [see JS: Object Constructor]
Array[…]new Array(…) [see JS: Array Constructor]
Functionfunction (…) {…}new Function(…) [see JS: Function Constructor]
Regex/…/…new RegExp(…) [see JS: RegExp Constructor]
Datenonenew Date(…) [see JS: Date Constructor]

There are many more constructors. Most standard builtin object has a constructor.

[see JavaScript Object Reference]

Primitive Value Wrapper Constructors

The following are constructors for primitive value's wrapper objects. (when you call a method on a primitive value, such as "a/b".split("/"), the string is temporarily converted to a string object. That's why it has methods.)

[see JS: Primitive Value]

Constructor for Primitive Values's Wrapper Objects
ObjectLiteral Expression SyntaxConstructor SyntaxNotes
Number{3, -3, 3.7, …}new Number(…) [see JS: Number Constructor]
String"…"new String("…") [see JS: String Constructor]
Boolean{true, false}new Boolean(…) [see JS: Boolean Constructor]

Use Literal Expression When Possible

For builtin objects, you should use the literal expression to create object whenever possible. Because:

  1. The constructor functions are often very complex, involves many arguments. Type and number of arguments may change its behavior.
  2. Some constructor can be called with new, some not. For example, the Symbol constructor cannot be called with new. [see JS: Symbol Tutorial]
  3. For some constructors, the behavior of calling it with new or without, is the same. For example, {• Object()Array()Function()RegExp()} are like that. But not others. For example, Date(). [see JS: Date Constructor]
  4. Some constructor are there simply to make the JavaScript language structure consistent, but is not designed for practical use. For example, Function().

User Defined Constructor

To define your own constructor:

For detail, see JS: Operator “new”

(Note: you cannot use arrow function to creat constructor. [see JS: Arrow Function])

Parenthesis Optional for Constructor Call with No Args

When a function is used as a constructor and without argument, the parenthesis are optional. For example, new F() and new F are equivalent.

Constructor Name Should Start with Capital Letter

By convention, functions designed to be used as constructor starts with a capital letter.

This is important, because, function designed as constructor have very different behavior than “normal” functions. For example, constructor function usually does not contain return statement, and usually will add/modify properties of “this binding”.

Function starting with capital letter lets you know whether it is designed to be called with new.

Though, you can write a function such that f(…) and new f(…) have the same behavior. (just have a return statement and have it return a object.) The builtin functions {• Object()Array()Function()RegExp()} are like that.

Do Not Confuse with Property Name “constructor”

Do not confuse constructor with property named “constructor”.

[see JS: Property Key “constructor”]

JS Constructor/Class

  1. “this” Binding
  2. What's Constructor?
  3. Property Key "prototype"
  4. Operator “new”
  5. Operator “instanceof”
  6. Property Key “constructor”
  7. Class
  8. Keyword “extends”
  9. Keyword “super”
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  1. JavaScript Basics
  2. true, false
  3. Operators
  4. Branch Control
  5. Loop
  6. Array Basics
  7. Object Basics
  8. RegExp Basics


  1. Value Types
  2. let
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  1. String Overview
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  4. String Escape Sequence
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  1. Define Function
  2. Arrow Function ⭐
  3. Function Params
  4. “arguments” Object
  5. Rest Params ⭐
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  7. Arg Destructure ⭐
  8. f Declaration vs Expression
  9. Closure
  10. f call apply bind
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  1. Property Overview
  2. Property Key
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  4. Create/Delete Property
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  12. Symbol

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  1. Object Overview
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  4. Prototype Chain
  5. Create Object
  6. Object Literal Expr
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  10. Prevent Adding Property
  11. Clone Object
  12. Test Object Equality
  13. Add Method to Prototype


  1. Understand JS Array
  2. Create Array
  3. Sparse Array
  4. Array-Like Object
  5. Array How-To


  1. “this” Binding
  2. What's Constructor?
  3. Property Key "prototype"
  4. Operator “new”
  5. Operator “instanceof”
  6. Property Key “constructor”
  7. Class
  8. Keyword “extends”
  9. Keyword “super”

Iterable ⭐

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  5. for-of Loop
  6. Spread Operator
  7. Generator
  8. Generator Function


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  4. Import/Export
  5. Promise
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  7. ES2015 Features
  8. How to Convert ES5 to ES2015
  9. Style Guide
  10. Semicolon Rules
  11. Exclamation Before Function
  12. How to Determine Strict Mode?
  13. x.y.z Associativity
  14. Function Chain
  15. Random Number, Range, Integer, Element
  16. Timing JS Code
  17. Range Function for Array
  18. Random Array Element, Randomize Array
  19. Tagged Template String
  20. String to Number
  21. Format Number

Object Reference

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  2. Object 🙼 Object.prototype
  3. Array 🙼 Array.prototype
  4. Function 🙼 Function.prototype
  5. String 🙼 String.prototype
  6. RegExp 🙼 RegExp.prototype 🙼 RegExp Syntax
  7. Date 🙼 Date.prototype
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  9. Map ⭐🙼 Map.prototype
  10. JSON
  11. Math
  12. Reflect
  13. Symbol ⭐🙼 Symbol.prototype
  14. Number 🙼 Number.prototype
  15. Boolean 🙼 Boolean.prototype

DOM How-To

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DOM Overview

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Basic DOM Methods

  1. Basic DOM Methods
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  10. How to Create Tooltip
  11. Falling Snow Effect
  12. JavaScript: Floating Box Following Scroll

DOM Misc

  1. Value of “this” in Event Handler
  2. Event Delegation
  3. Get Current Script Tag
  4. insertAfter Element
  5. Randomize List
  6. create Document Fragment
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