Constructor, operator "new"

Constructor function

Constructor functions technically are regular functions. There are two conventions though:

1. They are named with capital letter first.
2. They should be executed only with "new" operator.

function User(name) {
  this.name = name;
  this.isAdmin = false;
}

let user = new User("Jack");

alert(user.name); // Jack
alert(user.isAdmin); // false

When a function is executed with new, it does the following steps:

1. A new empty object is created and assigned to this.
2. The function body executes. Usually it modifies this, adds new properties to it.
3. The value of this is returned.

In other words, new User(...) does something like:

function User(name) {
  // this = {};  (implicitly)

  // add properties to this
  this.name = name;
  this.isAdmin = false;

  // return this;  (implicitly)
}

So let user = new User("Jack") gives the same result as:

let user = {
  name: "Jack",
  isAdmin: false
};

Now if we want to create other users, we can call new User("Ann"), new User("Alice") and so on. Much shorter than using literals every time, and also easy to read.

That’s the main purpose of constructors – to implement reusable object creation code.

Let’s note once again – technically, any function (except arrow functions, as they don’t have this) can be used as a constructor. It can be run with new, and it will execute the algorithm above. The “capital letter first” is a common agreement, to make it clear that a function is to be run with new.

new function() { … }

If we have many lines of code all about creation of a single complex object, we can wrap them in an immediately called constructor function, like this:

// create a function and immediately call it with new
let user = new function() {
  this.name = "John";
  this.isAdmin = false;

  // ...other code for user creation
  // maybe complex logic and statements
  // local variables etc
};

This constructor can’t be called again, because it is not saved anywhere, just created and called. So this trick aims to encapsulate the code that constructs the single object, without future reuse.

Constructor mode test: new.target

Inside a function, we can check whether it was called with new or without it, using a special new.target property.

It is undefined for regular calls and equals the function if called with new:

function User() {
  alert(new.target);
}

// without "new":
User(); // undefined

// with "new":
new User(); // function User { ... }

That can be used inside the function to know whether it was called with new, “in constructor mode”, or without it, “in regular mode”.

We can also make both new and regular calls to do the same, like this:

function User(name) {
  if (!new.target) { // if you run me without new
    return new User(name); // ...I will add new for you
  }

  this.name = name;
}

let john = User("John"); // redirects call to new User
alert(john.name); // John

This approach is sometimes used in libraries to make the syntax more flexible. So that people may call the function with or without new, and it still works.

Probably not a good thing to use everywhere though, because omitting new makes it a bit less obvious what’s going on. With new we all know that the new object is being created.

Return from constructors

Usually, constructors do not have a return statement. Their task is to write all necessary stuff into this, and it automatically becomes the result.

But if there is a return statement, then the rule is simple:

• If return is called with an object, then the object is returned instead of this.
• If return is called with a primitive, it’s ignored.

In other words, return with an object returns that object, in all other cases this is returned.

For instance, here return overrides this by returning an object:

function BigUser() {

  this.name = "John";

  return { name: "Godzilla" };  // <-- returns this object
}

alert( new BigUser().name );  // Godzilla, got that object

And here’s an example with an empty return (or we could place a primitive after it, doesn’t matter):

function SmallUser() {

  this.name = "John";

  return; // <-- returns this
}

alert( new SmallUser().name );  // John

Usually constructors don’t have a return statement. Here we mention the special behavior with returning objects mainly for the sake of completeness.

Omitting parentheses

By the way, we can omit parentheses after new:

let user = new User; // <-- no parentheses
// same as
let user = new User();

Omitting parentheses here is not considered a “good style”, but the syntax is permitted by specification.

Methods in constructor

Using constructor functions to create objects gives a great deal of flexibility. The constructor function may have parameters that define how to construct the object, and what to put in it.

Of course, we can add to this not only properties, but methods as well.

For instance, new User(name) below creates an object with the given name and the method sayHi:

function User(name) {
  this.name = name;

  this.sayHi = function() {
    alert( "My name is: " + this.name );
  };
}

let john = new User("John");

john.sayHi(); // My name is: John

/*
john = {
   name: "John",
   sayHi: function() { ... }
}
*/