JavaScript Christmas

The new Operator

A 5 minute read written by
Eirik Vigeland
21.12.2020

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You probably came here to read about something cutting edge in JavaScript, but today we’re going to do a deep dive into one of the fundamental operators of JavaScript, new.

I struggled with how the new operator fit into JavaScript when I first started out many years ago. Coming from object-oriented languages such as Java, it was strange to be able to use objects (and functions for that matter) before they were somehow instantiated (with new - although this is not really the case). Objects, in the object-oriented way, made a bit more sense to me when classes were introduced in ES5. Classes are really just syntactic sugar for creating objects in JavaScript, but unlike objects, classes will throw a TypeError if you attempt to use them without calling new. So today let's do a deep dive into how the new operator really works and try to demystify some of it's secrets.

Objects

Objects in JavaScript are just key/value pairs, which make up the properties of the objects. Values can be primitive, array, functions or any other object. Functions are called "methods" when they are used in objects. The easiest, and most common way to create an object in JavaScript is by using the literal notation known as "object initializer".

const person = {
  firstName: ‘John’,
  lastName: ‘Doe’,
  age: 23,
};

console.log(person.firstName); // John

Under the hood, the equivalent of using the literal notation is creating a new object and setting properties on that object. So even though you’ve never really explicitly created objects with new, it’s been done for you.

const person = new Object();
person.firstName = ‘Jane’;
person.lastName = ‘Doe’;
person.age = 26;

console.log(person.firstName); // Jane

Functions

So what about functions? What happens when you don’t use the new operator on functions? Functions in JavaScript are also objects, they are instances of the Function object.

function myFunction () {};
console.log(myFunction.constructor === Function); // true

Unlike the constructor of Object (new Object), calling the constructor of Function (new Function), is not the same as using function declarations (function () {}). You can read more to get a deeper understanding of the differences at MDN.

A function is defined in MDN as "a set of statements that performs a task or calculates a value", also called a procedure. Furthermore to qualify as a function, the procedure should return an output which has some relationship to the input of the function.

Unless a function has an explicit return, a default value is returned from the function. And here is where the new operator comes into play. The default return value of a function is undefined if the function is not a constructor function. A function is a constructor if it is called with the new operator. For a constructor function the default value is this. So for functions that are not called with the new operator. The statements of the function are executed and the explicit return value or undefined is returned. By using the new operator the function will return a newly created object which is bound to this.

function Person(firstName, lastName, age) {
  this.firstName = firstName;
  this.lastName = lastName;
  this.age = age;
}

const john = new Person(‘John’, ‘Doe’, 23);
console.log(john.firstName); // John

const jane = new Person(‘Jane’, ‘Doe’, 26);
console.log(jane.firstName); // Jane

It is important to note that this does not apply to arrow functions (const fun = () => {}). Arrow functions cannot be used as a constructor function and will yiled a TypeError when used in conjunction with new. One of the reasons is how arrow functions handle this, which will be bound to the scope where the arrow function is used. Therefore we cannot create new instances of arrow functions since the new operator will bind this of the function to the instance. You can read more about arrow functions and this at MDN.

Prototypes

The last thing we need to discuss are prototypes. JavaScript inheritance works by prototype chaining. I’m not going to reiterate all the properties of how prototypes and prototype chaining works. You can read more about it over at MDN.

In broad strokes an objects in JavaScript (almost) always inherits from the Object.prototype. Objects created with "object initializer" or new Object is no exception. Neither are objects created using constructor functions. But unlike "object initializer" and new Object, the constructor functions links the prototype of the returned object to the constructor as it’s parent prototype. The created object still inherits from the Object.prototype, but not as it’s parent prototype.

function Person(firstName, lastName, age) {
  this.firstName = firstName;
  this.lastName = lastName;
  this.age = age;
}

Back to our example of the Person constructor function, objects created with this function will first inherit from Person.prototype, then Object.prototype (and lastly null). When invoking a property on the object, JavaScript will walk this path to find the property. If it was not defined on the Person object, it would look at Object, and so on. This makes it easier for us to extend Person objects with new methods, which will apply to all instances of Person.

function Person(firstName, lastName, age) {
  this.firstName = firstName;
  this.lastName = lastName;
  this.age = age;
}

Person.prototype.fullName = function () {
  return `${this.firstName} ${this.lastName}`;
}

const jane = new Person(‘Jane’, ‘Doe’, 26);
console.log(jane.fullName()); // Jane Doe

Summary

I hope this gives you a better understanding of how the new operator works, in a nutshell. We have only scratched the surface of "the secret life of Objects" in JavaScript. And if you want to know more, I recommend you dive deeper into the documentation on MDN, read Eloquent JavaScript, or YDKJS series. To summarize, as documented on MDN, the new-operator:

  • Creates a blank, plain JavaScript object;
  • Links (sets the constructor of) the newly created object to another object by setting the other object as its parent prototype;
  • Passes the newly created object from Step 1 as the this context;
  • Returns this if the function doesn't return an object.
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