javascript-questions

1. What are Closure, Lexical Environment & Execution Context in javascript ?

Closure: In JavaScript, closures are created every time a function is created, at function creation time. To use a closure, simply define a function inside another function and expose it.

Lexical Environment: In JavaScript, every running function, code block, and the script as a whole have an associated object known as the Execution Context: When your code runs in the JavaScript Engine. Each statement of your code is executed in a certain Execution Context. A new Execution Context is created whenever function invocation is there.

closures

execution-context

2. What is Hoisting ? Are let & const are also hosted in JS ?

Hoisting: It is JavaScript’s default behavior of moving declarations to the top.

All declarations (var, let, const, function, function*, class) are “hoisted” in JavaScript.

A variable declared by let or const has a so-called temporal dead zone (TDZ): When entering its scope, it can’t be accessed (got or set) until execution reaches the declaration.

hoisting

3. Create Private variable in Javascript.

Using Classic prototype pattern

var Car = (function(){
  var speed = 80;
  function Car(model,wheels,seats){
    this.model = model;
    this.wheels = wheels;
    this.seats = seats;
  }
  Car.prototype.run = function(){
    console.log('car of model '+this.model+' having '+this.wheels+' wheels & '+this.seats+' seats is running at speed of '+speed+' km/hr');
  }
  return Car;
})();
var maruti = new Car('Maruti 800',4,5);
maruti.run();

Using Module pattern

var watch = (function(){
  var timer = 0;
  function increment(){
    timer++;
    console.log('timer value is: '+timer);
  }
  function decrement(){
    timer--;
    console.log('timer value is: '+timer);
  }
  return {
   increment: increment,
   decrement: decrement,
   };
})();
watch.increment();
watch.decrement();

Using Factory pattern

var CarFactory = function(model,wheels,seats){
   var speed = 80;
   function Car(model,wheels,seats){
      this.model = model;
      this.wheels = wheels;
      this.seats = seats;
   }
   Car.prototype.run = function(){
      console.log('car of model '+this.model+' having '+this.wheels+' wheels & '+this.seats+' seats is running at speed of '+speed+' km/hr');
   }
   Car.prototype.setSpeed = function(newSpeed){
      speed = newSpeed;
   }
   Car.prototype.getSpeed = function(){
      return speed;  
   }
   return new Car(model,wheels,seats)
};
var maruti = CarFactory('800',4,4);
var marcedeze = CarFactory('AMG',4,4);
maruti.setSpeed(100)
maruti.getSpeed() // 100
marcedeze.setSpeed(120)
mercedeze.getSpeed() // 120

What if we want to have variable speed to be same in all the factory objects ?

simply create speed variable outside, so it wont be part of closure which wont be part of new object.

var CarFactory = (function(){
   var speed = 80;
   return function(model,wheels,seats){
      function Car(model,wheels,seats){
         this.model = model;
         this.wheels = wheels;
         this.seats = seats;
      }
      Car.prototype.run = function(){
         console.log('car of model '+this.model+' having '+this.wheels+' wheels & '+this.seats+' seats is running at speed of '+speed+' km/hr');
      }
      Car.prototype.setSpeed = function(newSpeed){
         speed = newSpeed;
      }
      Car.prototype.getSpeed = function(){
         return speed;  
      }
      return new Car(model,wheels,seats)
   };
})();
var maruti = CarFactory('800',4,4);
var marcedeze = CarFactory('AMG',4,4);
maruti.setSpeed(100)
maruti.getSpeed() // 100
mercedeze.getSpeed() // 100