JavaScript Inheritance

inherit.js

function A(x) {
    this.x = x;
};

A.prototype = {
    log: function () {
        console.log('A', this.x);
    }
};

function B(x) {
    A.call(this, x); // or A.apply(this, arguments)
};

B.prototype = Object.create(A.prototype, {
    log: {
        value: function () {
            A.prototype.log.call(this);
            console.log('B', this.x * this.x);
        }
    }
});

B.prototype.constructor = B; // save constructor, otherwise B.prototype.constructor === Object

var b = new B(10);

b.log(); // => A, 10 \n B, 10

console.log(b instanceof A); // => true
console.log(b instanceof B); // => true