100 most asked JavaScript Interview Questions and Answers

Using Filter - It is possible to remove duplicates from an array in JavaScript by applying a filter to the same. To call the filter() method, three arguments are required. These are namely array as self, current element as elem, and index of the current element as index.

let language = ['JavaScript', 'Dart', 'Kotlin', 'Java', 'Swift', 'Dart']
function unique_array(arr) {
   let unique_array = arr.filter(function (elem, index, self) {
       return index == self.indexOf(elem);
   });
   return unique_array
}
console.log(unique_array(language));

// Logs [ 'JavaScript', 'Dart', 'Kotlin', 'Java', 'Swift' ]

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23. How to check if a value is an Array?

We can check if a value is an Array by using the Array.isArray() method available from the Array global object.

It returns true when the parameter pass to it is an Array otherwise false.

console.log(Array.isArray(5));  //logs false
console.log(Array.isArray("")); //logs false
console.log(Array.isArray()); //logs false
console.log(Array.isArray(null)); //logs false
console.log(Array.isArray({ length: 5 })); //logs false
console.log(Array.isArray([])); //logs true

If your environment does not support this method you can use the polyfill implementation.

function isArray(value){
   return Object.prototype.toString.call(value) === "[object Array]"
}

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24. Implement the Array.prototype.map() method.

As the MDN description of the Array.prototype.map method, the map() method creates a new array with the results of calling a provided function on every element in the calling array.

Syntax of map() method is

let newArray = arr.map(callback(currentValue[, index[, array]]) {
  // return element for newArray, after executing something
}[, thisArg]);

And here is the implementation of it

function map(arr, mapCallback) {
  // Check if the parameters passed are right.
  if (!Array.isArray(arr) || !arr.length || typeof mapCallback !== 'function') {
    return [];
    }
    else {
      let result = [];
      // Avoid mutating the original array.
      for (let i = 0, len = arr.length; i < len; i++) {
        result.push(mapCallback(arr[i], i, arr));
        // push the result of the mapCallback in the 'result' array
        }
        return result; // return the result array
    }
}

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25. Implement the Array.prototype.filter() method.

As the MDN description of the Array.prototype.filter method, the filter() method creates a new array with all elements that pass the test implemented by the provided function.

Syntax is

let newArray = arr.filter(callback(currentValue[, index[, array]]) {
// return element for newArray, if true
}[, thisArg]);

Implementations is

function filter(arr, filterCallback) {
// Check if the parameters passed are right.
if (!Array.isArray(arr) || !arr.length || typeof filterCallback !== 'function') {
  return [];
  }
  else {
    let result = [];
    // Avoid mutating the original array.
    for (let i = 0, len = arr.length; i < len; i++) {
      // check if the return value of the filterCallback is true or "truthy"
      if (filterCallback(arr[i], i, arr)) {
      // push the current item in the 'result' array if the condition is true
      result.push(arr[i]);
    }
  }
  return result; // return the result array
}
}

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26. Implement the Array.prototype.reduce() method.

The reduce() method executes a reducer function (that you provide) on each element of the array, resulting in single output value.
The reducer function takes four arguments:

Accumulator, Current Value, Current Index ,Source Array

Syntax is

arr.reduce(callback( accumulator, currentValue, [, index[, array]] )[, initialValue])

Implementation

function reduce(arr, reduceCallback, initialValue) {
// Check if the parameters passed are right.
if (!Array.isArray(arr) || !arr.length || typeof reduceCallback !== 'function'){
  return [];
}
else {
  // If no initialValue has been passed to the function we're gonna use the
  let hasInitialValue = initialValue !== undefined;
  let value = hasInitialValue ? initialValue : arr[0];
  // first array item as the initialValue, Start looping at index 1 if there is no
  // initialValue has been passed to the function else we start at 0 if there is an initialValue.
  for (let i = hasInitialValue ? 0 : 1, len = arr.length; i < len; i++) {
    // Then for every iteration we assign the result of the reduceCallback to the variable value.
    value = reduceCallback(value, arr[i], i, arr);
  }
  return value;
}
}

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27. What is a name function in JavaScript?

A named function declares a name as soon as it is defined. It can be defined using function keyword as :

function named() {
   // write code here
}

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28. Can you assign an anonymous function to a variable and pass it as an argument to another function?

Yes! An anonymous function can be assigned to a variable.
It can also be passed as an argument to another function.

Exmample is

let show = function () {
  console.log('Anonymous function');
};
show();

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29. What is the arguments object?

The arguments object is a collection of parameter values pass in a function.
It's an Array-like object because it has a length property and we can access individual values using array indexing notation arguments[1]
But it does not have the built-in methods in an array forEach, reduce, filter and map.
It helps us know the number of arguments pass in a function.

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30. Can you convert arguments object into an array?

Yes, We can convert the arguments object into an array using the Array.prototype.slice.
```
function one() {
 return Array.prototype.slice.call(arguments);
}
```
However, if there is a need to automatically execute a function at the place where it is given and not be called again, then anonymous functions can be used. Such functions have no name. So the name.

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31. Does arguments object work on ES6 arrow functions?

No, the arguments object does not work on ES6 arrow functions.

function one() {
   return arguments;
}
const two = function () {
   return arguments;
}
const three = function three() {
   return arguments;
}
const four = () => arguments;
four(); // Throws an error  - arguments is not defined

When we invoke the function four it throws a ReferenceError: arguments is not defined error.

We can solve this problem if your environment supports the rest syntax.

const four = (...args) => args;

This puts all parameter values in an array automatically.

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32. How to create an object without a prototype?

We can create an object without a prototype using the Object.create method.

const o1 = {};
console.log(o1.toString());
// logs [object Object] get this method to the Object.prototype

const o2 = Object.create(null);
// the first parameter is the prototype of the object "o2" which in this case will be null specifying we don't want any prototype
console.log(o2.toString());
// throws an error o2.toString is not a function

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33. What are the scopes of a variable in JavaScript?

The scope of a variable is the region of your program in which it is defined. JavaScript variable will have only two scopes.

Global Variables − A global variable has global scope which means it is visible everywhere in your JavaScript code.
Local Variables − A local variable will be visible only within a function where it is defined. Function parameters are always local to that function.

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34. What is the purpose of this in JavaScript?

The JavaScript this keyword refers to the object it belongs to.

This has different values depending on where it is used.

In a method, this refers to the owner object
In a function, this refers to the global object.

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35. What is Callback in JavaScript?

A callback is a plain JavaScript function passed to some method as an argument or option.
It is a function that is to be executed after another function has finished executing, hence the name ‘callback’.
In JavaScript, functions are objects, So functions can take functions as arguments, and can be returned by other functions.

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36. How does typeOf Operator work?

The typeof operator is used to get the data type of its operand.
The operand can be either a literal or a data structure such as a variable, a function, or an object.
It is a unary operator that is placed before its single operand, which can be of any type.
Its value is a string indicating the data type of the operand.

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37. Explain equality in JavaScript.

JavaScript has both strict and type–converting comparisons:

Strict comparison (e.g., ===) checks for value equality without allowing coercion
Abstract comparison (e.g. ==) checks for value equality with coercion allowed.

var a = "42";
var b = 42;
a == b; // true
a === b; // false

Some simple equality rules:

If either value (aka side) in a comparison could be the true or false value, avoid == and use ===.
If either value in a comparison could be of these specific values (0, "", or [] -- empty array), avoid == and use ===.
In all other cases, you're safe to use ==.
- Not only it is safe, but in many cases it simplifies your code in a way that improves readability.

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

== is the abstract equality operator while === is the strict equality operator.
The == operator will compare for equality after doing any necessary type conversions.
The === operator will not do type conversion, so if two values are not the same type === will simply return false.

When using ==, funky things can happen, such as:

1 == "1"; // true
1 == [1]; // true
1 == true; // true
0 == ""; // true
0 == "0"; // true
0 == false; // true

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39. What is ECMAScript?

ECMAScript is a standard for making scripting languages which means that JavaScript follows the specification changes in ECMAScript standard because it is the blueprint of JavaScript.
ECMAScript standardized by the ECMA International standards organization in the ECMA-262 and ECMA-402 specifications.
Read more about ECMAScript here.

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40. What are the new features in ES6 or ECMAScript 2015?

Arrow Functions
Classes
Template Strings
Enhanced Object literals
Object Destructuring
Promises
Generators
Modules
Symbol
Proxies
Sets
Default Function parameters
Rest and Spread Operators
Block Scoping with let and const

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41. What does use strict do?

use strict is a ES5 feature in JavaScript that makes our code in Strict Mode in functions or entire scripts.
Strict Mode helps us avoid bugs early on in our code and adds restrictions to it.

Let look on restrictions that Strict Mode gives us.

Assigning or Accessing a variable that is not declared.

function returnA() {
"use strict";
a = 3245;
return a;
}

Assigning a value to a read-only or non-writable global variable

"use strict";
var NaN = NaN;
var undefined = undefined;
var Infinity = "and beyond";

Deleting an undeletable property


"use strict";
const obj = {};
Object.defineProperty(obj, 'x', {
     value : '1'
}); 

delete obj.x;

Duplicate parameter names


"use strict";

function someFunc(a, b, b, c){

}

Creating variables with the use of the eval function


"use strict";

eval("var x = 1;");
console.log(x); //Throws a Reference Error x is not defined

The default value of this will be undefined


"use strict";

function showMeThis(){
 return this;
}
showMeThis(); //returns undefined

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Originally posted on AnkitKumar.Dev

100 most asked JavaScript Interview Questions and Answers - Part 2

JavaScript Interview Questions and Answers Series

Questions

22. How to remove duplicates from an array?

23. How to check if a value is an Array?

24. Implement the Array.prototype.map() method.

25. Implement the Array.prototype.filter() method.

26. Implement the Array.prototype.reduce() method.

27. What is a name function in JavaScript?

28. Can you assign an anonymous function to a variable and pass it as an argument to another function?

29. What is the arguments object?

30. Can you convert arguments object into an array?

31. Does arguments object work on ES6 arrow functions?

32. How to create an object without a prototype?

33. What are the scopes of a variable in JavaScript?

34. What is the purpose of this in JavaScript?

35. What is Callback in JavaScript?

36. How does typeOf Operator work?

37. Explain equality in JavaScript.

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

39. What is ECMAScript?

40. What are the new features in ES6 or ECMAScript 2015?

41. What does use strict do?

Further Reading