... because this is the right answer when asked about JavaScript inheritance

inheritance.js

function inherit(proto) {
  function F() {};
  F.prototype = proto;
  return new F;
  
  // Note: use `Object.create` instead of `inherit` if you do not need to support old browsers
}

// create any "class"
function A() {
  // create A-specific instance members
  this.propOfA = 'a';
}
A.prototype.performA = function(){ console.log('performA'); };

// create another "class"
function B() {
  // inherit instance members
  A.call(this);
  
  // instance members of B
  this.propOfB = 'b';
}

// inherit from A -- pick one
  // for old browsers (and new ones)
  B.prototype = inherit(A.prototype);
  // for new browsers (better option)
  B.prototype = Object.create(A.prototype);

// create B-specific methods
B.prototype.performB = function(){ console.log('performB'); };

// usage
var a = new A();
var b = new B();

a instanceof A; // true
a instanceof B; // false
b instanceof A; // true
b instanceof B; // true