Arrays and Objects JavaScript Interview Questions

 

1. Explain about Arrays in JavaScript?

An Array holds an ordered sequence of items.

• They are used to store multiple values in a single variable.
• The 
  typeof
   operator in JavaScript returns 
  object
   for arrays.

2. How to compare two arrays?

In JavaScript, to compare two arrays,

• The length of both the arrays should be the same. The type and value of each item in one array should be compared with the counterpart in another array.

• JavaScript provides a function

  JSON.stringify()
  in order to convert an object or array into JSON string. By converting into JSON string we can directly check if the strings are equal or not.

3. How can you create an Array in JavaScript?

Creating an Array using the Array Literal

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let x = [];

let y = [1, 2, 3, 4, 5];

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Creating an Array using the JavaScript Keyword new

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const evenNumbers = new Array(2, 4, 6);

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4. Explain about reduce, map, filter methods?

reduce():

The

reduce

method executes a reducer function (that we provide) on each element of the array. It passes in the return value from the calculation on the preceding element.

The result is a single value. It is the result of running the reducer across all elements of the array.

Example:

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const numbers = [1, 2, 3, 4];

const reducer = (previousValue, currentValue) => previousValue + currentValue;

console.log(numbers.reduce(reducer)); // Output: 10

 

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In each step, the reducer function adds the current array element to the result from the previous step until there are no more elements to add.

map():

The

map

method creates a new array with the results of calling the provided function for each array element in order.

Example:

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const numbers = [1, 2, 3, 4];

const result = numbers.map((number) => number * number);

console.log(result); // Output: [1, 4, 9, 16]

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filter():

• The 
  filter
   method creates a new array filled with all elements that pass the test (provided as a function).
• A new array with the elements that pass the test will be returned. If no elements pass the test, an empty array will be returned.

Example:

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const numbers = [1, -2, 3, -4];

const positiveNumbers = numbers.filter((number) => number > 0);

console.log(positiveNumbers); // Output: [1, 3]

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5. What is the use of the push method in JavaScript?

The

push

method adds new items to the end of the array.

6. Write a sample program for map and filter?

Sample program for map:

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const numbers = [1, 2, 3, 4];

const doubled = numbers.map((item) => item * 2);

console.log(doubled); // Output: [2, 4, 6, 8]

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Sample program for filter:

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const numbers = [1, 2, 3, 4];

const evenNumbers = numbers.filter((item) => item % 2 === 0);

console.log(evenNumbers); // Output: [2, 4]

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7. What is the use of a map method?

• Instead of manually iterating over the array using a loop, we can simply use the built-in 
  map
   method.
• The 
  map
   method allows you to iterate over an array and modify its elements using a callback function. The callback function will then be executed on each of the array's elements.

8. Explain about the array method slice?

The

slice

method returns the selected elements in an array, as a new array.

It returns a portion between the specified start index and end index(end index not included) of an array into a new array.

Syntax:

arr.slice(startIndex, endIndex)

Example:

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let fruits = ["Banana", "Orange", "Lemon", "Apple", "Mango"];

let citrus = fruits.slice(1, 3);

console.log(citrus); // Output: ["Orange", "Lemon"]

console.log(fruits); // Output: ["Banana", "Orange", "Lemon", "Apple", "Mango"]

 

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9. How do you empty an array??

There are several ways to empty an array. The simplest one is to reassign the variable to an empty array.

Examples:

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let array = [1, 2, 3];

// Assigning to an empty array

array = [];

// Setting its length property to 0

array.length = 0;

// Using splice with the array's length

array.splice(0, array.length);

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10. What are the most useful JavaScript array methods?

Some of the most useful JavaScript array methods are:

• push()

  • The
    push
    method adds elements to an array at the last position.

• pop()

  • The
    pop
    method removes the last element of an array and returns it.

• forEach()

  • The
    forEach
    method executes a provided function once for each array element. It always returns undefined.

• map()

  • The
    map
    method creates a new array with the results of calling the provided function for every array element.

• filter()

  • The
    filter
    method creates a new array filled with all elements that pass the test (provided as a function).

• splice()

  • The
    splice
    method changes the contents of an array by removing or replacing existing elements and/or adding new elements.

• slice()

  • The
    slice
    method returns a portion between the specified start index and end index(end index not included) of an array into a new array.

• shift()

  • The
    shift
    method removes the first element of an array.

• unshift()

  • The
    unshift
    method adds one or more elements to the beginning of an array.

• sort()

  • The
    sort
    method sorts the items of an array and returns the sorted array. The default sort order is ascending.

11. What are the differences between slice and splice?

Slice	Splice
Doesn't modify the original array(immutable)	Modifies the original array(mutable)
Returns the subset of original array	Returns the deleted elements as array
Used to pick the elements from array	Used to insert or delete elements to/from array

12. Explain about pop and shift methods?

pop():

The

pop

method removes the last element from an array and returns it. The array on which

pop

method is called will be changed.

Example:

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let fruits = ["Apple", "Orange", "Banana", "Strawberry"];

console.log(fruits.pop()); // Output: Strawberry

console.log(fruits); // Output: ["Apple", "Orange", "Banana"]

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shift():

The

shift

method removes an item from the beginning of an array and returns the removed item. The array on which

shift

method is called will be changed.

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let fruits = ["Apple", "Orange", "Banana", "Strawberry"];

console.log(fruits.shift()); // Output: Apple

console.log(fruits); // Output: ["Orange", "Banana", "Strawberry"]

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13. What is the difference between map and forEach methods?

Both of them iterate through the elements of an array. The difference is that the

map

creates a new array while

forEach

doesn’t.

14. Which JavaScript methods are used to search for an element in an array?

indexOf():

The

indexOf

method returns the first index at which a given element can be found in an array. It returns -1 if the element does not exist in the array.

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const array = [100, 121, 35, 5];

const indexOfThirtyFive = array.indexOf(35);

console.log(indexOfThirtyFive); // Output: 2

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includes():

The

includes

method determines whether an array includes a certain value and returns true or false as appropriate.

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const array = [10, 11, 3, 20, 5];

const includesThirtyFive = array.includes(35);

console.log(includesThirtyFive); // Output: false

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filter():

We can use the

filter

method to find elements in an array that meet a certain condition.

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const array = [1, 12, 3, 24, 35];

const equalToThirtyFive = array.filter((element) => element === 35);

console.log(equalToThirtyFive); // Output: [35]

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find():

We use the

find

method to find the first element that meets a certain condition. Just like the

filter

method, it takes a callback as an argument and returns the first element that meets the callback condition.

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const array = [110, 1, 35, 0, 25];

const equalToThirtyFive = array.find((element) => element === 35);

console.log(equalToThirtyFive); // Output: 35

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15. What is the purpose of the splice method?

The

splice

method changes the contents of an array.

Remove existing items

Syntax:

arr.splice(Start, Delete Count)

• Start: Starting Index
• Delete Count: Number of items to be removed, starting from the given index

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let myArray = [5, "six", 2, 8.2];

myArray.splice(2, 2);

console.log(myArray); // Output: [5, "six"]

let deletedItems = myArray.splice(2, 2);

console.log(deletedItems); // Output: [2, 8.2]

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The

splice()

method returns an array containing the deleted items.

Add new items

Syntax:

arr.splice(Start, Delete Count, Item1, Item2 ... )

Here the

Item1, Item2 ...

are the items to be added, starting from the given index.

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let myArray = [5, "six", 2, 8.2];

myArray.splice(2, 0, "one", false);

console.log(myArray); // Output: [5, "six", "one", false, 2, 8.2]

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Replace existing items

Syntax:

arr.splice(Start, Delete Count, Item1, Item2 ... )

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let myArray = [5, "six", 2, 8.2];

myArray.splice(2, 1, true);

console.log(myArray); // Output: [5, "six", true, 8.2]

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16. How to sort the numbers in an array?

By default, the sort method sorts elements alphabetically. To sort numerically just add the Compare Function that handles numeric sorts:

Compare Function:

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function(a, b){

return a - b

}

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When the

sort()

function compares two values, it sends the values to the compare function and sorts the values according to the returned (negative, zero, positive) value.

• If the result is negative, a is sorted before b.

• If the result is positive, b is sorted before a.

• If the result is 0, no changes are done with the sort order of the two values.

Example:

• The compare function compares all the values in the array, two values at a time (a, b).

• When comparing 40 and 100, the

  sort
  method calls the compare function(40, 100).

• The function calculates 40 - 100 (a - b), and since the result is negative (-60), the

  sort
  method will sort 40 as a value lower than 100.

Example:

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let numArray = [40, 100, 99];

numArray.sort(function (a, b) {

return a - b;

});

console.log(numArray); // Output: [40, 99, 100]

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17. What is an object and explain object creation?

An Object is a collection of properties, and a property is an association between a name (or key) and a value.

In JavaScript, there are four ways to create an object:

Object Literals:

An object literal also called an object initializer, is a comma-separated set of paired names and values.

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const car = {

model: "BMW",

color: "red",

price: 2000,

};

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New operator or constructor:

If we call a function using a new operator, the function acts as a constructor and returns an object.

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function Car(model, color) {

this.model = model;

this.color = color;

}

let c1 = new Car("BMW", "red");

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Object.create method

We can also create new objects using the

Object.create

method, which allows us to specify the prototype object and the properties.

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let Car = {

model: "BMW",

color: "red",

};

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We can use the Car object as a prototype to create another object, as shown below:

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let ElectricCar = Object.create(Car);

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Class

We can use the

class

keyword to create a class in JavaScript instead of a function constructor, and use the

new

operator to create an instance.

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class Car {

constructor(maker, price) {

this.maker = maker;

this.price = price;

}

getInfo() {

console.log(this.maker + " costs : " + this.price);

}

}

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We can use the Car class to create objects as shown below:

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let car1 = new Car("BMW", 100);

car1.getInfo();

let car2 = new Car("Audi", 150);

car2.getInfo();

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18. What are object prototypes?

A prototype is a blueprint of an object. The prototype allows us to use properties and methods on an object even if the properties and methods do not exist on the current object.

All JavaScript objects inherit properties from a prototype.

• Date objects inherit properties from the Date prototype.

• Math objects inherit properties from the Math prototype.

• Array objects inherit properties from the Array prototype.

Example:

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let arr = [];

arr.push(2);

console.log(arr); // Output: [2]

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19. How to add key-value pair to an object in JavaScript?

There are two possible solutions to add new properties to an object.

Using dot notation: This solution is useful when you know the name of the property.

Example:

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let object = {

key1: value1,

key2: value2,

};

object.key3 = "value3";

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Using square bracket notation: This solution is useful when the name of the property is dynamically determined.

Example:

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let object = {

key1: value1,

key2: value2,

};

obj["key3"] = "value3";

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20. What is Object Destructuring?

To unpack properties from Objects, we use Object Destructuring. The variable name should match with the key of an object.

Example:

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let person = {

firstName: "Rahul",

lastName: "Attuluri",

age: 28,

};

let { age } = person;

console.log(age); // Output: 28

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21. In the arrow functions, what is the significance of 'this' keyword?

In Arrow functions,

this

depends on two aspects:

• When the code is defined
• Context

Arrow function inherits

this

from the context in which the code is defined.

22. What is the use of 'this' keyword in JavaScript?

The

this

keyword allows us to reuse functions in different execution contexts. It means, a function once defined can be invoked for different objects using the

this

keyword.