Understanding JavaScript Constructors

The following is a guest post by Faraz Kelhini. Some of this stuff is out of my comfort zone, so I asked Kyle Simpson to tech check it for me. Kyle's answer (which we did during an Office Hours session) was very interesting. It was: 1) This article is technically sound. JavaScript doesn't really have classes in a traditional sense and this is the way most people shoehorn them in. 2) We may want to stop shoehorning them in. JavaScript has objects and we can use them in the way they are intended to do the same kinds of things. Kyle calls it OLOO (Objects Linked to Other Objects). Here's an intro. I'd think there is value in learning about both.

Having a good understanding of constructors is crucial to truly understand the JavaScript language. Unlike many other languages, JavaScript doesn't support classes, but it has constructors to bring similar functionality to JavaScript. In this tutorial, we will explore constructors in detail and see how JavaScript utilizes them to make objects.
Constructors are like regular functions, but we use them with the "new" keyword. There are two types of constructors: native (aka built-in) constructors like Array and Object, which are available automatically in the execution environment at runtime; and custom constructors, which define properties and methods for your own type of object.

A constructor is useful when you want to create multiple similar objects with the same properties and methods. It’s a convention to capitalize the name of constructors to distinguish them from regular functions. Consider the following code:

function Book() { 
  // unfinished code
var myBook = new Book();

The last line of the code creates an instance of Book and assigns it to a variable; even though the Book constructor doesn't do anything, myBook is still an instance of it. As you can see there is no difference between this function and regular functions except that we call it with the new keyword and the function name is capitalized.

Determining the Type of an Instance

To find out whether an object is an instance of another one, we use the instanceof operator:

myBook instanceof Book    // true
myBook instanceof String  // false

Note that if the right side of the instanceof operator isn’t a function, it will throw an error:

myBook instanceof {};
// TypeError: invalid 'instanceof' operand ({})

Another way to find the type of an instance is using the constructor property. All object instances have a constructor property that point to the constructor function that created it.

Consider the following code fragment:

myBook.constructor == Book;   // true

Since the constructor property of myBook points to Book the result is true. All objects inherit a constructor property from their prototype:

var s = new String("text");
s.constructor === String;      // true

"text".constructor === String; // true

var o = new Object();
o.constructor === Object;      // true

var o = {};
o.constructor === Object;      // true

var a = new Array();
a.constructor === Array;       // true

[].constructor === Array;      // true

Although checking constructor property can be used to check the type of an instance, it is recommended to only use the instanceof operator for this purpose, because the constructor property might be overwritten, so it cannot be a reliable method for checking the type of an instance.

Custom Constructor Functions

A constructor is like a cookie cutter to make more than one object with similar features. In other words, the benefit of using a constructor is that it makes it easy to create multiple objects with the same properties and methods.

Consider the following code:

function Book(name, year) {
  this.name = name;
  this.year = '(' + year + ')';

The book constructor expects two parameters: name and year; when it is called with the new keyword it assigns the received parameters to the name and year property of the current instance so the constructor can be used to create objects with initialized name and year properties:

var firstBook = new Book("Pro AngularJS", 2014);
var secondBook = new Book("Secrets Of The JavaScript Ninja", 2013); 
var thirdBook = new Book("JavaScript Patterns", 2010);
console.log(firstBook.name, firstBook.year);           
console.log(secondBook.name, secondBook.year);           
console.log(thirdBook.name, thirdBook.year);  

This code logs the following in the console:

As you can see, we can quickly build a large number of different book objects by invoking the Book constructor with different arguments. This is exactly the same pattern that JavaScript uses in its built-in constructors like Array() and Date().

Object.defineProperty Function

The Object.defineProperty() can be used inside of a constructor to help perform all necessary property setup. Consider the following constructor:

function Book(name) { 
  Object.defineProperty(this, "name", { 
      get: function() { 
        return "Book: " + name;       
      set: function(newName) {            
        name = newName;        
      configurable: false     

var myBook = new Book("Single Page Web Applications");
console.log(myBook.name);    // Book: Single Page Web Applications

// we cannot delete the name property because "configurable" is set to false
delete myBook.name;    
console.log(myBook.name);    // Book: Single Page Web Applications

// but we can change the value of the name property
myBook.name = "Testable JavaScript";
console.log(myBook.name);    // Book: Testable JavaScript

In this code we used accessor properties inside the Object.defineProperty(). Accessor properties don’t include any properties or methods, but they define a getter to call when the property is read, and a setter to call when the property is written to.

A getter is expected to return a value, while a setter receives the value being assigned to the property as an argument. This constructor allows us to set or change the name property of instances, but we are not allowed to delete it, and when we get the value of name, the getter prepends the string "Book: " to the name and returns it.

Object Literal Notations are Preferred to Constructors

The JavaScript language has nine built-in constructors: Object(), Array(), String(), Number(), Boolean(), Date(), Function(), Error() and RegExp(). When creating values we are free to use either object literals or constructors, but object literals are not only easier to read but also faster to run because they can be optimize at parse time. Whenever we need simple objects it's best to stick with literals:

// a number object
// numbers have a toFixed() method
var obj = new Object(5);
obj.toFixed(2);     // 5.00

// we can achieve the same result using literals
var num = 5;
num.toFixed(2);     // 5.00

// a string object
// strings have a slice() method 
var obj = new String("text");
obj.slice(0,2);     // "te"

// same as above
var string = "text";
string.slice(0,2);  // "te"

As you can see there's hardly any difference between these object literals and constructors and we can still call methods on literals. It's because when we call a method on a literal, behind the scene JavaScript converts the literal to a temporary object so that it's possible to use object methods for primitive values, then JavaScript discards the temporary object.

Using the "new" Keyword is Essential

It's important to remember to use the new keyword before all constructors, if we accidentally forget "new" we will be modifying the global object instead of the newly created object. Consider the following example:

function Book(name, year) {
  this.name = name;
  this.year = year;

var myBook = Book("js book", 2014);  
console.log(myBook instanceof Book);  
console.log(window.name, window.year);

var myBook = new Book("js book", 2014);  
console.log(myBook instanceof Book);  
console.log(myBook.name, myBook.year);

When we call the Book constructor without new keyword, in fact we are just calling a function without a return statement and "this" inside the Book constructor will be equal to Window (instead of myBook), in other words we have unintentionally created two global variables which causes the code to produce unintended results, but when we call the function with the "new" keyword the context is switched from global (window) to the instance, therefore "this" points to myBook. Here is what the above code logs in browser console:

Note that in strict mode this code throws an error because strict mode protects us from accidentally calling a constructor without the new keyword.

Scope-Safe Constructors

Since a constructor is just a function, it can be called without the new keyword, but this leads to unexpected results and errors especially for inexperienced programmers. The solution to this problem is scope-safe constructors. We can call Scope-safe constructors with or without new keyword and they return the same result in either form.

Most of built-in constructors, such as Object, Regex and Array, are scope-safe. They use a pattern to determine whether the constructor is called with new or not.

If new isn't used, a proper instance of the object is created by calling the constructor again with new keyword. Consider the following code:

function Book(name) { 
  if (!(this instanceof Book)) { 
    // the constructor was called without "new".
    return new Book(name);

So using this pattern we can easily rewrite a scope-safe version of our constructor:

function Book(name, year) { 
  if (!(this instanceof Book)) { 
    return new Book(name, year);
  this.name = name;
  this.year = year;

var person1 = new Book("js book", 2014);
var person2 = Book("js book", 2014);
console.log(person1 instanceof Book);    // true
console.log(person2 instanceof Book);    // true

The fact that "this" is an instance of the custom type allows us to determine if new is not used as soon as the constructor begins to execute and run the constructor again with new keyword.


JavaScript has no class statement, which is baffling for coders who are used to languages with a class statement; However, JavaScript has constructors to bring similar functionality. Constructors are just regular functions which are used with new keyword. They come in handy when we need to make multiple similar objects with the same properties and methods.