Introduction to the JavaScript Function type

In JavaScript, all functions are objects. They are the instances of the Function type. Because functions are objects, they have properties and methods like other objects.

Functions properties

Each function has two important properties: length and prototype.

• The length property determines the number of named arguments specified in the function declaration.
• The prototype property references the actual function object.

See the following example:

function add(x, y) {
    return x + y;
}

console.log(add.length);// 2
console.log(add.prototype);// Object{} 

The add() function accepts two arguments x and y. Therefore, the length property returns two.

new.target

Typically, you call a function normally like this:

let result = add(10,20);
console.log(result);// 30 

Also, you can call a function with new keyword as a constructor:

let obj = new add(10,20); 

ES6 introduced the new.target pseudo-property that allows you to detect whether a function or constructor was called using the new operator.

If a function is called normally, the new.target is undefined. However, if the function is called using the new keyword as a constructor, the new.target return a reference to the constructor.

For example:

function add(x, y) {
  console.log(new.target);
  return x + y;
}

let result = add(10, 20);
let obj = new add(10, 20); 

Output:

undefined
[Function: add] 

By using the new.target, you can control how a function will be called.

For example, to prevent the add() function from being called with the new keyword as a constructor, you can throw an error by checking the new.target like this:

function add(x, y) {
  if (new.target) {
    throw 'The add function cannot be called as a constructor';
  }
  return x + y;
}

let obj = new add(10, 20);
console.log(obj); 

Function methods: apply, call, and bind

A function object has three important methods: apply(), call() and bind().

The apply() and call() methods

The apply() and call() methods call a function with a given this value and arguments.

The difference between the apply() and call() is that you need to pass the arguments to the apply() method as an array-like object, whereas you pass the arguments to the call() function individually. For example:

let cat = { type: 'Cat', sound: 'Meow' };
let dog = { type: 'Dog', sound: 'Woof' };

const say = function (message) {
  console.log(message);
  console.log(this.type + ' says ' + this.sound);
};

say.apply(cat, ['What does a cat say?']);
say.apply(dog, ['What does a dog say?']); 

Output:

What does a cat sound?
Cat says Meow
What does a dog sound?
Dog says Woof

In this example:

First, declare two objects cat and dog with two properties:

let cat = { type: 'Cat', sound: 'Meow' };
let dog = { type: 'Dog', sound: 'Woof' }; 

Second, define the say() function that accepts one argument:

const say = function (message) {
  console.log(message);
  console.log(this.type + ' says ' + this.sound);
}; 

Third, call the say() function via the apply() method:

say.apply(cat, ['What does a cat say?']); 

In this example, the first argument of the  apply() method is the cat object. Therefore, the this object in the say() function references the cat object.

Fourth, call say() function and pass the dog object:

say.apply(dog, ['What does a dog say?']); 

In this example, the this in the say() function reference the dog object.

The call() method like the apply() method except for the way you pass the arguments to the function:

say.call(cat, 'What does a cat say?');
say.call(dog, 'What does a dog say?'); 

The bind() method

The bind() method creates a new function instance whose this value is bound to the object that you provide. For example:

First, define an object named car:

let car = {
    speed: 5,
    start: function() {
        console.log('Start with ' + this.speed + ' km/h');
    }
}; 

Then, define another object named aircraft:

let aircraft = {
    speed: 10,
    fly: function() {
        console.log('Flying');
    }
}; 

The aircraft has no start() method. To start an aircraft, you can use the bind() method of the start() method of the car object:

let taxiing = car.start.bind(aircraft); 

In this statement, we change the this value inside the start() method of the car object to the  aircraft object.  The bind() method returns a new function that is assigned to the taxiing variable.

Now, you can call the start() method via the taxiing variable:

taxiing();

It will show the following message:

Start with 10 km/h 

The following uses the call() method to call the start() method on the aircraft object:

car.start.call(aircraft); 

As you can see, the bind() method creates a new function that you can execute later while the call() method executes the function immediately. This is the main difference between the bind() and call() methods.

Technically, the aircraft object borrows the start() method of the car object via the bind(), call() or apply() method.

For this reason, the bind(), call(), and apply() methods are also known as borrowing functions.

Summary

• All functions are instances of the Function type, which are the objects that have properties and methods.
• A function has two important properties: length and prototype.
• A function also has three important methods: call(), apply(), and bind().