Question

Q1) How is this possible?!

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

var fred = new employer('Fred');

Isn’t employer a function rather than a class?

Q2) What does

employer.prototype

signify?

and finally

Q3) How does this work?

employer.prototype.salary = null;
fred.salary = 2000;

Can someone please give simple explanation? Please 🙂

Answer 1

As you asked for a simple explanation, here we go:

Q1) This is possible because JavaScript functions are first-class (i.e. they are objects, and you can create new objects).

Q2) A prototype is an object from which other objects inherit properties. Properties of an objects prototype will be available to all instances of that object.

Q3) This means that all instances of employer will have a property salary with the value null. The instance of employer called fred simply overrides that initial null value with 2000. It’s the same salary property though (when you use a property, the scope chain is searched until a match is found. In this case, a match is found on the object prototype).

Quick example:

function employer(name) { this.name = name;};
employer.prototype.salary = 1000;

var fred = new employer('Fred');
var jim = new employer('Jim');

console.log(fred.salary) //Prints 1000 because employer.prototype.salary == 1000
fred.salary = 2000;
console.log(fred.salary) //Prints 2000 because fred's salary has been changed
console.log(jim.salary) //Prints 1000 because jim's salary is still referencing the prototype

fred.age = 30;
console.log(fred.age) //Prints 30
console.log(jim.age) //Prints undefined (age is a property of fred, not employer.prototype