Constructor functions

Declaring a constructor function

Constructor functions are functions designed to construct a new object. Within a constructor function, the keyword this refers to a newly created object which values can be assigned to. Constructor functions "return" this new object automatically.

function Cat(name) {
  this.name = name;
  this.sound = "Meow";
}

Constructor functions are invoked using the new keyword:

let cat = new Cat("Tom");
cat.sound; // Returns "Meow"

Constructor functions also have a prototype property which points to an object whose properties are automatically inherited by all objects created with that constructor:

Cat.prototype.speak = function() {
  console.log(this.sound);
}

cat.speak(); // Outputs "Meow" to the console

Objects created by constructor functions also have a special property on their prototype called constructor, which points to the function used to create them:

cat.constructor // Returns the `Cat` function

Objects created by constructor functions are also considered to be "instances" of the constructor function by the instanceof operator:

cat instanceof Cat // Returns "true"

Remarks

Constructor functions are actually just regular functions, there's nothing special about them. It's only the new keyword which causes the special behavior shown in the examples above. Constructor functions can still be called like a regular function if desired, in which case you would need to bind the this value explicitly.