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JavaScript Questions

Below is a compiled list of JavaScript interview questions that will help you conquer that upcoming interview of yours!

  1. What will the code below output to the console and why?

    var myObject = {
      foo: "bar",
      func: function() {
       var self = this;
       console.log("outer func: this.foo = " + this.foo);
       console.log("outer func: self.foo = " + self.foo);
       (function() {
        console.log("inner func: this.foo = " + this.foo);
        console.log("inner func: self.foo = " + self.foo);
       }());
      }
    };
    myObject.func();

  2. What do you understand by Closures in JavaScript?

  3. What will be the output of the code below?

    var x = { foo : 1};
    var output = (function(){
       delete x.foo;
       return x.foo;
      })();

    console.log(output);

  4. Explain few difference between null, undefined or undeclared JavaScript variable?

  5. What are escape Sequences?

  6. What is SetTimeout()?

  7. What is the value of !'bang'?

  8. How would you use a closure to create a private counter?

  9. What is NaN? What is its type? How can you reliably test if a value is equal to NaN?

  10. How to empty an array in JavaScript?

  11. The following recursive code will cause a stack overflow if the array list is too large. How can you fix this and still retain the recursive pattern?


    var list = readHugeList();
    var nextListItem = function() {
    var item = list.pop();

    if (item) {
    // process the list item...
    nextListItem(); }
    };

  12. What are the characteristics of JavaScript ‘Strict Mode’?

  13. What is the difference between == and ===?

  14. Difference between “var” and “let” Keywords?

  15. Between JavaScript and an ASP script, which is faster?

  16. What will be the output of the code below ?

    var str = "abcd";
    str[0] = "b";
    console.log(str);

  17. What is hoisting in JavaScript ?

  18. What is Memoization?

  19. Does JavaScript support automatic type conversion?

  20. What is generator in JS?

  21. What do you mean by Event Bubbling and Event Capturing?

  22. Determine the output of following code on console ?

    1. var x = 1;
    2. function outer(){
    3.   var y = 2;
    4.   x++;
    5.   function inner(){
    6.     x++;
    7.     y++;
    8.     console.log(x,y);
    9.   }
    10.   return inner;
    11. }
    12. var innerFun = outer();
    13. innerFun();
    14. outer();
    15. innerFun();

  23. What is DOM? What is the use of document object?

  24. Explain the role of deferred scripts in JavaScript.

  25. What's the difference between Function.prototype.apply and Function.prototype.call?

  26. What are JavaScript Promises?

  27. What are screen objects?

  28. What is the purpose of ‘This’ operator in JavaScript?

  29. What are JavaScript Promises?

  30. What is the ‘Strict’ mode in JavaScript and how can it be enabled?

  31. What is a “closure” in JavaScript?

  32. What will be the output of the following code:

    for (var i = 0; i < 5; i++) { setTimeout(function() { console.log(i); }, i * 1000 ); }

    Explain your answer. How could the use of closures help here?

  33. How to define an anonymous function?

  34. Explain the difference between escape() and unescape() functions?

  35. What are the different ways to define a variable in JavaScript? And when to pick which one to use?

  36. What is Callback?

  37. Explain window.onload and onDocumentReady?

  38. Please explain Self Invoking Function and its syntax.

  39. What is event loop in JavaScript ?

  40. What are the different types of errors in JavaScript?

  41. What is an IIFE (Immediately Invoked Function Expression) in Javascript?

  42. What is called Variable typing in Javascript?

  43. What is a difference between call, apply and bind in JavaScript?

  44. What would following code return?

    console.log(typeof typeof 1);