Javascript Interview

This documents contains the most actual and important questions for Javascript developer position. It will helps you to compose question list for your own interview or prepare to inverview.

Table of contents:

• Inheritance in Javascript:
  • Write Object.create polyfill
  • What is functional inherence pattern? How to create protected methods and private propreties?
  • What will output the following code
• Closure in Javascript:
  • What is Closure in Javascript?
  • What will output the following code
  • Resolving example of problem code
  • Write nextID function
• Bind, apply and call function methods
  • Write sum function
• Hoisting in Javascript:
  • What is a hoisting in Javascript
  • What will output the following code
  • What will output the following code

Inheritance in Javascript

Question i1: Write Object.create polyfill

For implementing use this kata please.

Question i2: What is functional inherence pattern? How to create protected methods and private propreties?

Question i3: What will output the following code and why:

var baseObject = {
  prop: "Base value",
  getProp: function() {
    return this.prop;
  }
};

var otherObject = Object.create(baseObject);

otherObject.prop = "Other value";

console.log(otherObject.getProp());
delete otherObject.prop;
console.log(otherObject.getProp()); 

Answer

Console will output

Other value
Base value

Closure in Javascript

Question c1: What is Closure in Javascript?

MDN definition: Closures are functions that refer to independent (free) variables (variables that are used locally, but defined in an enclosing scope). In other words, these functions 'remember' the environment in which they were created.

Javascript.isSexy: A closure is an function that has access to the outer (enclosing) function’s variables-scope chain.

Question c2: What will output the following code and why:

function outerFunction() {
    var flag = undefined;

    function innerFunction() {
      if (true) {
        flag = true;
      } else {
        var flag = false;
      }

      console.log(flag);
    }

    console.log(flag);
    innerFunction();
}

outerFunction();

Answer

Console will output the followings:

undefined
true

Question c3: Resolving example of problem code

What will output these example?

for (var i=0; i < 10; i++){
    setTimeout(function(){
        console.log(i);
    }, 1000);
}

How to fix it to output numbers from 0 to 9?

Answer

Ther're several ways to resolve code above.

1. By creating a IIFE (Immediately Invoked Function Expression):

for (var i=0; i < 10; i++){
    setTimeout((function(param){
        console.log(i);
    })(i), 1000);
}

2. By using ES6 feature, by using let syntax particularly:

for (let i=0; i < 10; i++){
    setTimeout(function(){
        console.log(i);
    }, 1000);
}

3. By binding console.log function:

for (var i=0; i < 10; i++){
    setTimeout(console.log.bind(console, i), 1000);
}

Question c4: Write nextID function that will generate ID incrementally after each calling

There's an example of function output:

nextID(); // output: 1
nextID(); // output: 2
nextID(); // output: 3

Answer

We should use closure in IIFE:

var nextID = (function() {
   var id = 0;
   return function() {
   		console.log(++id);
   }
})();

Bind, apply and call function methods

Question b1: Write sum function

This function must meet conditions:

typeof sum(1) === 'function'
typeof sum(1)(2) === 'function'
typeof sum(1)(2)() === 'number'

sum(1)() === 1
sum(1,2)(3)() == 6
sum(1,2)(3,4)() === 10

Answer

function sum (fn) {
  var sumArguments = Array.prototype.slice.call(arguments);
    
  return function sumInner() {
      if (arguments.length) {
          sumArguments = sumArguments.concat(Array.prototype.slice.call(arguments));

          return sumInner;
      } else {
          return sumArguments.reduce(function(total, arg) {
            return total + arg;
          }, 0);
      }
  };
}

Hoisting in Javascript

Question h1: Explain what is hoisting in Javascript

Answer

W3School Hoisting is JavaScript's default behavior of moving declarations to the top. In JavaScript, a variable can be declared after it has been used. In other words; a variable can be used before it has been declared.

Question h2: What will output the following code and why:

var a = 1; 
function bar() { 
    if (!a) { 
        var a = 10; 
    } 
    console.log(a); 
} 
bar();

Answer

Console will output :

10

Question h3: What will output the following code and why:

var a = 1;
function b() {
    a = 10;
    return;
    function a() {}
} 
b(); 
console.log(a);

Answer

Console will output :

1

Contributing

I would be thankful for your issues and pull requests