Foolproof constructor for objects

a. description.md

One of javascript pitfalls is object constructor: if you forgot to add new before your function, it will not be called as constructor, but as usual function.

b. now.js

var Human = function Human() {
  this.age = 5;
  this.askAge = function() {
    return "My age is " + age;
  }
}

//Problem lies here: forgot to add `new`
var Mike = Human();

//and Mike is undefined
console.log(Mike); // undefined

//but now window has age and askAge() properties
console.log(window.age); // 5

c. foolproof.js

var Human = function Human() {
  //and there it is.
  if (this === window) return new Human(arguments);
  
  this.age = 5;
  this.askAge = function() {
    return "My age is " + age;
  }
}

//And our problem goes away
var Mike = Human();

//and now window has age and askAge() properties
console.log(age); // ReferenceError: age is not defined