JavaScript Interview Questions and Answers 70 JavaScript Interview Questions

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This is a long one, so bear with me for a second or an hour. In every answer for every question there's an arrow up ↑ link that lets you go back to the List of Questions so that you don't waste time scrolling up and down.

The Questions

• 1. What's the difference between undefined and null?
• 2. What does the && operator do?
• 3. What does the || operator do?
• 4. Is using the + or unary plus operator the fastest way in converting a string to a number?
• 5. What is the DOM?
• 6. What is Event Propagation?
• 7. What's Event Bubbling?
• 8. What's Event Capturing?
• 9. What's the difference between event.preventDefault() and event.stopPropagation() methods?
• 10. How to know if the event.preventDefault() method was used in an element?
• 11. Why does this code obj.someprop.x throw an error?
• 12. What is event.target ?
• 13. What is event.currentTarget?
• 14. What's the difference between == and ===?
• 15. Why does it return false when comparing two similar objects in JavaScript?
• 16. What does the !! operator do?
• 17. How to evaluate multiple expressions in one line?
• 18. What is Hoisting?
• 19. What is Scope?
• 20. What are Closures?
• 21. What are the falsy values in JavaScript?
• 22. How to check if a value is falsy?
• 23. What does "use strict" do?
• 24. What's the value of this in JavaScript?
• 25. What is the prototype of an object?
• 26. What is an IIFE, what is the use of it?
• 27. What is the use Function.prototype.apply method?
• 28. What is the use Function.prototype.call method?
• 29. What's the difference between Function.prototype.apply and Function.prototype.call?
• 30. What is the usage of Function.prototype.bind?
• 31. What is Functional Programming and what are the features of JavaScript that makes it a candidate as a functional language?
• 32. What are Higher Order Functions?
• 33. Why are functions called First-class Objects?
• 34. Implement the Array.prototype.map method by hand.
• 35. Implement the Array.prototype.filter method by hand.
• 36. Implement the Array.prototype.reduce method by hand.
• 37. What is the arguments object?
• 38. How to create an object without a prototype?
• 39. Why does b in this code become a global variable when you call this function?
• 40. What is ECMAScript?
• 41. What are the new features in ES6 or ECMAScript 2015?
• 42. What's the difference between var, let and const keywords?
• 43. What are Arrow functions?
• 44. What are Classes?
• 45. What are Template Literals?
• 46. What is Object Destructuring?
• 47. What are ES6 Modules?
• 48. What is the Set object and how does it work?
• 49. What is a Callback function?
• 50. What are Promises?
• 51. What is async/await and How does it work?
• 52. What's the difference between Spread operator and Rest operator?
• 53. What are Default Parameters?
• 54. What are Wrapper Objects?
• 55. What is the difference between Implicit and Explicit Coercion?
• 56. What is NaN? and How to check if a value is NaN?
• 57. How to check if a value is an Array?
• 58. How to check if a number is even without using the % or modulo operator?
• 59. How to check if a certain property exists in an object?
• 60. What is AJAX?
• 61. What are the ways of making objects in JavaScript?
• 62. What's the difference between Object.seal and Object.freeze methods?
• 63. What's the difference between the in operator and the hasOwnProperty method in objects?
• 64. What are the ways to deal with Asynchronous Code in JavasScript?
• 65. What's the difference between a function expression and function declaration?
• 66. How many ways can a function be invoked?
• 67. What is memoization and what's the use it?
• 68. Implement a memoization helper function.
• 69. Why does typeof null return object? How to check if a value is null?
• 70. What does the new keyword do?

1. What's the difference between undefined and null?.

↑ Before understanding the differences between undefined and null we must understand the similarities between them.

• They belong to JavaScript's 7 primitive types.

 let primitiveTypes = ['string','number','null','undefined','boolean','symbol', 'bigint'];

• They are falsy values. Values that evaluated to false when converting it to boolean using Boolean(value) or !!value.

   console.log(!!null); //logs false
   console.log(!!undefined); //logs false

   console.log(Boolean(null)); //logs false
   console.log(Boolean(undefined)); //logs false

Ok, let's talk about the differences.

• undefined is the default value of a variable that has not been assigned a specific value. Or a function that has no explicit return value ex. console.log(1). Or a property that does not exist in an object. The JavaScript engine does this for us the assigning of undefined value.

  let _thisIsUndefined;
  const doNothing = () => {};
  const someObj = {
    a : "ay",
    b : "bee",
    c : "si"
  };

  console.log(_thisIsUndefined); //logs undefined
  console.log(doNothing()); //logs undefined
  console.log(someObj["d"]); //logs undefined

• null is "a value that represents no value". null is value that has been explicitly defined to a variable. In this example we get a value of null when the fs.readFile method does not throw an error.

  fs.readFile('path/to/file', (e,data) => {
     console.log(e); //it logs null when no error occurred
     if(e){
       console.log(e);
     }
     console.log(data);
   });