Constructor Functions in JavaScript

A constructor function is a function used as a blueprint to create many similar objects. By habit its name starts with a capital letter, and you call it with the new keyword.

function Person(name, age) {
  this.name = name;
  this.age = age;
  this.greet = function () {
    return "Hi, I am " + this.name;
  };
}

const p1 = new Person("Robin", 25);
const p2 = new Person("Maria", 30);

console.log(p1.name);
console.log(p2.greet());

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What does new do?

• Creates a new empty object.
• Sets this inside the function to that object.
• Runs the function body, which adds properties to this.
• Returns the object.

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Shared methods with prototype

In the example above, every object gets its own copy of greet, which wastes memory. Put shared methods on the prototype, and all objects use the same function.

function Person(name) {
  this.name = name;
}

Person.prototype.greet = function () {
  return "Hi, I am " + this.name;
};

const p1 = new Person("Robin");
const p2 = new Person("Maria");
console.log(p1.greet());
console.log(p1.greet === p2.greet);

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instanceof

console.log(p1 instanceof Person);
console.log(p1.constructor === Person);

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Summary

• A constructor function is a blueprint, called with new.
• Use this to set properties, and the prototype for shared methods.
• Classes are the modern syntax for the same idea.
• Next: The this keyword.

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