Constructors

A constructor is a function that does initialization of an object when the object is created. Arguments are passed to the constructor function when you use the new keyword. There are two types of constructors, one defined as a function, and one defined with the constructor method in a class.

The Object Constructor Function

To use a constructor function to create objects, simply define a JavaScript function with any number of arguments. Inside the function, the keyword this is used as a placeholder for the object being created. If the constructor function is used by itself, without the new statement, this will have no value.

It’s preferred to capitalize a constructor function.

Example

function Car(make, model, year) {
  this.make = make;
  this.model = model;
  this.year = year;
}

var car1 = new Car('Chevy', 'Blazer', 2015);
var car2 = new Car('Ford', 'Taurus', 2018);

console.log('Car 1 is a ' + car1.year + ' ' + car1.make + ' ' + car1.model);
// Output: Car 1 is a 2015 Chevy Blazer

console.log('Car 2 is a ' + car2.year + ' ' + car2.make + ' ' + car2.model);
// Output: Car 2 is a 2018 Ford Taurus

The Class constructor Method

When defining a class in javascript, there is a special constructor method defined within it. Like the object constructor function, the constructor method can be defined with any number of arguments, and the this keyword can be used as a placeholder for the object being created.

Example

class Car {
  constructor(make, model, year) {
    this.make = make;
    this.model = model;
    this.year = year;
  }
}

var car1 = new Car('Chevy', 'Blazer', 2015);
var car2 = new Car('Ford', 'Taurus', 2018);

console.log('Car 1 is a ' + car1.year + ' ' + car1.make + ' ' + car1.model);
// Output: Car 1 is a 2015 Chevy Blazer

console.log('Car 2 is a ' + car2.year + ' ' + car2.make + ' ' + car2.model);
// Output: Car 2 is a 2018 Ford Taurus