21 Essential JavaScript Interview Questions by Codementor Nishant Kumar
What is the difference between undefined and not defined in JavaScript? Question 1:
What will be the output of the code below? var y = 1; if (function f(){}) { y += typeof f; } console.log(y); Question 2:
What is the drawback of creating true private methods in JavaScript? Question 3:
What is a “closure” in JavaScript? Provide an example. Question 4:
Write a mul function which will produce the following outputs when invoked:
 javascript console.log(mul(2)(3)(4)); // output : 24 console.log(mul(4)(3)(4)); // output : 48 Question 5:
How do you empty an array in JavaScript? Question 6:
How do you check if an object is an array or not? Question 7:
What will be the output of the following code? var output = (function(x){ delete x; return x; })(0); console.log(output); Question 8:
What will be the output of the following code? var x = 1; var output = (function(){ delete x; return x; })(); console.log(output); Question 9:
What will be the output of the following code? var x = { foo : 1}; var output = (function(){ delete x.foo; return x.foo; })(); console.log(output); Question 10:
What will be the output of the following code? var x = { foo : 1}; var Employee = { company: 'xyz' } var emp1 = Object.create(Employee); delete emp1.company console.log(emp1.company); Question 11:
What is undefined x 1 in JavaScript? Question 12:
What will be the output of the following code? Question 13: var trees = ["xyz","xxxx","test","ryan","apple"]; delete trees[3]; console.log(trees.length);
What will be the output of the following code? var bar = true; console.log(bar + 0); console.log(bar + "xyz"); console.log(bar + true); console.log(bar + false); Question 14:
What will be the output of the following code? var x = { foo : 1}; var z = 1, y = z = typeof y; console.log(y); Question 15:
What will be the output of the following code? // NFE (Named Function Expression var foo = function bar(){ return 12; }; typeof bar(); Question 16:
What is the difference between the following two functions? Question 17: var foo = function(){ // Some code }; function bar(){ // Some code };
What is function hoisting in JavaScript? Question 18:
What will be the output of the following code? Question 19: var salary = "1000$"; (function () { console.log("Original salary was " + salary); var salary = "5000$"; console.log("My New Salary " + salary); })();
What is the instanceof operator in JavaScript? What would be the output of the code below? Question 20: function foo(){ return foo; } new foo() instanceof foo;
If we have a JavaScript associative array Question 21: var counterArray = { A : 3, B : 4 }; counterArray["C"] = 1; How can we calculate length of its counterArray?
Did you know how to answer all 21 questions? Feel free to check your answers & view detailed explanations here!

21 Essential JavaScript Interview Questions

  • 1.
    21 Essential JavaScript InterviewQuestions by Codementor Nishant Kumar
  • 2.
    What is thedifference between undefined and not defined in JavaScript? Question 1:
  • 3.
    What will bethe output of the code below? var y = 1; if (function f(){}) { y += typeof f; } console.log(y); Question 2:
  • 4.
    What is thedrawback of creating true private methods in JavaScript? Question 3:
  • 5.
    What is a“closure” in JavaScript? Provide an example. Question 4:
  • 6.
    Write a mulfunction which will produce the following outputs when invoked:
 javascript console.log(mul(2)(3)(4)); // output : 24 console.log(mul(4)(3)(4)); // output : 48 Question 5:
  • 7.
    How do youempty an array in JavaScript? Question 6:
  • 8.
    How do youcheck if an object is an array or not? Question 7:
  • 9.
    What will bethe output of the following code? var output = (function(x){ delete x; return x; })(0); console.log(output); Question 8:
  • 10.
    What will bethe output of the following code? var x = 1; var output = (function(){ delete x; return x; })(); console.log(output); Question 9:
  • 11.
    What will bethe output of the following code? var x = { foo : 1}; var output = (function(){ delete x.foo; return x.foo; })(); console.log(output); Question 10:
  • 12.
    What will bethe output of the following code? var x = { foo : 1}; var Employee = { company: 'xyz' } var emp1 = Object.create(Employee); delete emp1.company console.log(emp1.company); Question 11:
  • 13.
    What is undefinedx 1 in JavaScript? Question 12:
  • 14.
    What will bethe output of the following code? Question 13: var trees = ["xyz","xxxx","test","ryan","apple"]; delete trees[3]; console.log(trees.length);
  • 15.
    What will bethe output of the following code? var bar = true; console.log(bar + 0); console.log(bar + "xyz"); console.log(bar + true); console.log(bar + false); Question 14:
  • 16.
    What will bethe output of the following code? var x = { foo : 1}; var z = 1, y = z = typeof y; console.log(y); Question 15:
  • 17.
    What will bethe output of the following code? // NFE (Named Function Expression var foo = function bar(){ return 12; }; typeof bar(); Question 16:
  • 18.
    What is thedifference between the following two functions? Question 17: var foo = function(){ // Some code }; function bar(){ // Some code };
  • 19.
    What is functionhoisting in JavaScript? Question 18:
  • 20.
    What will bethe output of the following code? Question 19: var salary = "1000$"; (function () { console.log("Original salary was " + salary); var salary = "5000$"; console.log("My New Salary " + salary); })();
  • 21.
    What is theinstanceof operator in JavaScript? What would be the output of the code below? Question 20: function foo(){ return foo; } new foo() instanceof foo;
  • 22.
    If we havea JavaScript associative array Question 21: var counterArray = { A : 3, B : 4 }; counterArray["C"] = 1; How can we calculate length of its counterArray?
  • 23.
    Did you knowhow to answer all 21 questions? Feel free to check your answers & view detailed explanations here!