JavaScript Array Interview Questions and Answers

JavaScript Array Interview Questions and Answers contains the list of top 50 array based questions that are frequently asked in interviews. The questions list are divided based on difficulty levels (Basic, Intermediate, and Advanced). This guide covers fundamental concepts, common problems, and practical solutions, helping you master array manipulation techniques and impress interviewers with your knowledge and skills. Ideal for developers preparing for technical interviews.

Many top tech companies like - Google, Meta, Microsoft, Amazon, and Others include JavaScript array questions in their interview processes to assess candidates' problem-solving skills and technical proficiency. Master essential array concepts to enhance your coding skills and impress interviewers.

JavaScript array is a data structure used to store multiple values in a single variable. This article contains a set of questions that are mostly asked during the interviews. There is a detailed answer coupled with code examples which can help you prepare adequately enough.

Our Top 50 JavaScript Array interview questions have been picked strategically so that you can be ready for an interview. These queries will enable you to demonstrate your expertise and impress recruiters from major multinational corporations (MNCs).

JavaScript Array Interview Questions and Answers - Basic Level

1. What is an array in JavaScript? How do you declare an array?

An array in JavaScript is a data structure that allows you to store multiple values in a single variable. Arrays can hold values of any data type, and each value is accessed using an index, with the first index being 0.

// Declaring an array with three elements
let fruits = ['apple', 'banana', 'cherry'];

2. How do you access the elements of an array?

Elements in an array are accessed using their index. The index of the first element is 0, the second element is 1, and so on.

let fruits = ['apple', 'banana', 'cherry'];
console.log(fruits[0]);
console.log(fruits[1]);
console.log(fruits[2]);

apple
banana
cherry

3. How can you add elements to an array?

Elements can be added to an array using the push method to add to the end, and the unshift method to add to the beginning.

let fruits = ['apple', 'banana'];
fruits.push('cherry'); // Adds 'cherry' to the end
console.log(fruits);
fruits.unshift('mango'); // Adds 'mango' to the beginning
console.log(fruits); 

[ 'apple', 'banana', 'cherry' ]
[ 'mango', 'apple', 'banana', 'cherry' ]

4. How can you remove elements from an array?

Elements can be removed from an array using the pop method to remove from the end, and the shift method to remove from the beginning.

let fruits = ['apple', 'banana', 'cherry'];
fruits.pop(); // Removes 'cherry' from the end
console.log(fruits); 
fruits.shift(); // Removes 'apple' from the beginning
console.log(fruits);

[ 'apple', 'banana' ]
[ 'banana' ]

5. What is length property of an array?

The length property of an array returns the number of elements in the array. It is a dynamic property, which means it automatically updates when the array changes.

let fruits = ['apple', 'banana', 'cherry'];
console.log(fruits.length);

3

6. How do you check if a variable is an array?

To check if a variable is an array, you can use the Array.isArray method. This method returns true if the variable is an array, and false otherwise.

let fruits = ['apple', 'banana'];
console.log(Array.isArray(fruits));
let notAnArray = 'apple';
console.log(Array.isArray(notAnArray));

true
false

7. How do you iterate over an array?

You can iterate over an array using a for loop. This allows you to access each element in the array by its index.

let fruits = ['apple', 'banana', 'cherry'];
for (let i = 0; i < fruits.length; i++) {
  	console.log(fruits[i]);
}

apple
banana
cherry

8. How do you use the forEach method?

The forEach method is used to execute a provided function once for each array element. It takes a callback function as an argument, which is called with each element, its index, and the entire array.

The forEach method does not return a new array and does not modify the original array.

let fruits = ['apple', 'banana', 'cherry'];
fruits.forEach((fruit, index) => {
  	console.log(`Fruit at index ${index} is ${fruit}`);
});

Fruit at index 0 is apple
Fruit at index 1 is banana
Fruit at index 2 is cherry

9. What is the difference between for...of and for...in loops in arrays?

The for...of loop iterates over the values of an array, whereas the for...in loop iterates over the keys (indexes) of an array. The for...of loop is generally preferred for arrays because it directly accesses the elements, while the for...in loop is more suitable for iterating over object properties.

10. How do you concatenate two arrays?

You can concatenate two arrays using the concat method or the spread operator (...).

The concat method returns a new array that is the combination of the two arrays, while the spread operator allows you to create a new array by spreading the elements of both arrays.

let fruits = ['apple', 'banana'];
let moreFruits = ['cherry', 'mango'];

// Using concat method
let allFruits = fruits.concat(moreFruits);
console.log(allFruits);

// Using spread operator
let allFruits = [...fruits, ...moreFruits];
console.log(allFruits);

JavaScript Array Interview Questions and Answers - Intermediate Level

1. Explain the map method and provide an example.

The map method creates a new array populated with the results of calling a provided function on every element in the calling array. It does not modify the original array but returns a new array with the transformed values.

The map method is often used for data transformation tasks, such as converting an array of numbers to their squares or extracting specific properties from an array of objects.

2. How does the filter method work?

The filter method creates a new array with all elements that pass the test implemented by the provided function. It calls the function for each element in the array and includes the element in the new array if the function returns true.

The filter method does not modify the original array and is useful for extracting a subset of elements based on certain criteria, such as finding all even numbers or filtering out invalid entries.

let numbers = [1, 2, 3, 4, 5];
let evenNumbers = numbers.filter(num => num % 2 === 0);
console.log(evenNumbers);

[ 2, 4 ]

3. Describe the reduce method with an example.

The reduce method executes a reducer function (that you provide) on each element of the array, resulting in a single output value. The reducer function takes two arguments: the accumulator (which accumulates the callback's return values) and the current value (the current element being processed).

The reduce method is commonly used for summing up elements, finding averages, or performing other cumulative operations.

let numbers = [1, 2, 3, 4];
let sum = numbers.reduce((accumulator, currentValue) => accumulator + currentValue, 0);
console.log(sum);

10

4. What is the purpose of the find method?

The find method returns the value of the first element in the array that satisfies the provided testing function. If no elements satisfy the testing function, undefined is returned.

let numbers = [1, 2, 3, 4, 5];
let foundNumber = numbers.find(num => num > 3);
console.log(foundNumber);

4

5. How do you use the some method?

The some method tests whether at least one element in the array passes the test implemented by the provided function. It returns a Boolean value.

let numbers = [1, 2, 3, 4, 5];
let hasEvenNumber = numbers.some(num => num % 2 === 0);
console.log(hasEvenNumber);

true

6. Explain the every method with an example.

The every method tests whether all elements in the array pass the test implemented by the provided function. It returns a Boolean value.

let numbers = [1, 2, 3, 4, 5];
let allEvenNumbers = numbers.every(num => num % 2 === 0);
console.log(allEvenNumbers)

false

7. What does the includes method do?

The includes method determines whether an array includes a certain value among its entries, returning true or false as appropriate.

It can also accept a second argument to specify the position in the array at which to begin the search. The includes method is useful for checking the presence of an element in an array.

8. How do you use the indexOf method?

The indexOf method returns the first index at which a given element can be found in the array, or -1 if it is not present. It can also accept a second argument to specify the index to start the search from.

let fruits = ['apple', 'banana', 'cherry'];
console.log(fruits.indexOf('banana'));
console.log(fruits.indexOf('mango')); 

1
-1

9. What is the slice method used for?

The slice method returns a shallow copy of a portion of an array into a new array object selected from start to end (end not included) where start and end represent the index of items in that array.

The original array is not modified.

10. Explain the splice method with an example.

The splice method changes the contents of an array by removing or replacing existing elements and/or adding new elements in place. It modifies the original array and returns an array of the removed elements.

let fruits = ['apple', 'banana', 'cherry'];
fruits.splice(1, 1, 'mango', 'grape');
console.log(fruits);

[ 'apple', 'mango', 'grape', 'cherry' ]

11. How do you use the join method?

The join method creates and returns a new string by concatenating all of the elements in an array (or an array-like object), separated by commas or a specified separator string. It is useful for creating a string from the elements of an array, such as converting an array of words into a sentence.

let fruits = ['apple', 'banana', 'cherry'];
let fruitString = fruits.join(', ');
console.log(fruitString);

apple, banana, cherry

12. What is array destructuring and how does it work?

Array destructuring is a syntax that allows you to unpack values from arrays or properties from objects into distinct variables. It provides a convenient way to extract multiple elements from an array and assign them to variables in a single statement. You can also set default values for the variables in case the array does not contain enough elements.

let fruits = ['apple', 'banana', 'cherry'];
let [firstFruit, secondFruit] = fruits;
console.log(firstFruit);
console.log(secondFruit);

let [first, second, third, fourth = 'default'] = fruits;
console.log(fourth)

apple
banana
default

13. How do you copy an array?

You can copy an array using the spread operator (...) or the Array.from method. Both methods create a shallow copy of the original array. The spread operator is concise and widely used, while Array.from is useful when you need to convert array-like objects or iterable objects to arrays.

let fruits = ['apple', 'banana', 'cherry'];

// Using spread operator
let fruitsCopy1 = [...fruits];
console.log(fruitsCopy1);

// Using Array.from method
let fruitsCopy2 = Array.from(fruits);
console.log(fruitsCopy2);

[ 'apple', 'banana', 'cherry' ]
[ 'apple', 'banana', 'cherry' ]

14. What are array-like objects and how do you convert them to arrays?

Array-like objects are objects that have a length property and indexed elements but do not have array methods like push, pop, etc. Common examples of array-like objects are the arguments object in functions and NodeList objects returned by DOM methods. You can convert array-like objects to arrays using Array.from or the spread operator (...).

15. Explain the flat method.

The flat method creates a new array with all sub-array elements concatenated into it recursively up to the specified depth. It is used to flatten nested arrays.

By default, flat flattens one level deep, but you can specify the depth as an argument to flatten deeper levels of nested arrays.

16. How does the flatMap method work?

The flatMap method first maps each element using a mapping function, then flattens the result into a new array. It is similar to map followed by flat with a depth of 1. The flatMap method is useful for scenarios where you need to both transform and flatten an array in a single step.

17. What is the from method and how is it used?

The Array.from method creates a new, shallow-copied Array instance from an array-like or iterable object. It is useful for converting other structures, such as NodeLists, strings, or sets, into arrays. You can also provide a mapping function as the second argument to Array.from to transform each element of the new array.

let string = 'hello';
let stringArray = Array.from(string);
console.log(stringArray);

let set = new Set([1, 2, 3]);
let setArray = Array.from(set);
console.log(setArray); 

[ 'h', 'e', 'l', 'l', 'o' ]
[ 1, 2, 3 ]

18. Explain the fill method with an example.

The fill method changes all elements in an array to a static value, from a start index (default 0) to an end index (default array length). It returns the modified array. The fill method is useful for initializing an array with a specific value or resetting elements in an existing array.

let numbers = [1, 2, 3, 4];
numbers.fill(0, 2, 4); // Fills with 0 from index 2 to 4 (exclusive)
console.log(numbers); 

[ 1, 2, 0, 0 ]

19. What does the sort method do and how can you customize it?

The sort method sorts the elements of an array in place and returns the sorted array.

By default, it sorts elements as strings in ascending order. This can lead to incorrect results when sorting numbers.

To customize the sort order, you can provide a compare function that defines the sort order. The compare function should return a negative value if the first argument is less than the second, zero if they're equal, and a positive value if the first argument is greater than the second.

20. What is the reverse method and how is it used?

The reverse method reverses the order of the elements in an array in place. The first element becomes the last, and the last element becomes the first. The reverse method modifies the original array and is useful when you need to reverse the order of elements, such as reversing a list of numbers or words.

JavaScript Array Interview Questions and Answers - Advanced Level

1. How do you merge two arrays and remove duplicates?

You can merge two arrays and remove duplicates by using the Set object, which automatically removes duplicate values.

let array1 = [1, 2, 3];
let array2 = [2, 3, 4];
let mergedArray = [...new Set([...array1, ...array2])];
console.log(mergedArray);

[ 1, 2, 3, 4 ]

2. Explain the concept of array buffer and typed arrays in JavaScript.

Array buffers are used to represent a generic, fixed-length binary data buffer.

Typed arrays are array-like views of an underlying binary data buffer. They provide a mechanism for accessing raw binary data.

let buffer = new ArrayBuffer(16);
let view = new Uint8Array(buffer);
console.log(view.length);

16

3. How do you sort an array of objects by a property value?

To sort an array of objects by a property value, you can use the sort method with a compare function that compares the desired property values.

let users = [
  { name: 'John', age: 30 },
  { name: 'Jane', age: 25 },
  { name: 'Peter', age: 35 }
];
users.sort((a, b) => a.age - b.age);
console.log(users);

[
  { name: 'Jane', age: 25 },
  { name: 'John', age: 30 },
  { name: 'Peter', age: 35 }
]

4. What is the difference between deep copy and shallow copy of an array?

A shallow copy of an array copies the array structure but not the objects it contains. Changes to objects in the original array affect the copied array and vice versa.

A deep copy copies the array and all nested objects, so changes to the original array do not affect the copied array.

let original = [{ a: 1 }, { b: 2 }];
let shallowCopy = [...original];
let deepCopy = JSON.parse(JSON.stringify(original));

original[0].a = 10;
console.log(shallowCopy[0].a); 
console.log(deepCopy[0].a);

10
1

5. How can you find the intersection of two arrays?

The intersection of two arrays can be found by using the filter method to return elements that are present in both arrays.

let array1 = [1, 2, 3, 4];
let array2 = [3, 4, 5, 6];
let intersection = array1.filter(value => array2.includes(value));
console.log(intersection);

[ 3, 4 ]

6. What are sparse arrays and how do they differ from dense arrays?

Sparse arrays have gaps between elements (i.e., some indices are empty), while dense arrays have elements at every index. Sparse arrays consume less memory for the gaps but can have performance implications.

let denseArray = [1, 2, 3];
let sparseArray = [1, , 3]; 
console.log(denseArray.length);
console.log(sparseArray.length); 
console.log(sparseArray[1]);

3
3
undefined

7. How do you remove duplicates from an array?

Duplicates can be removed from an array using a Set, which automatically removes duplicates.

let numbers = [1, 2, 2, 3, 4, 4, 5];
let uniqueNumbers = [...new Set(numbers)];
console.log(uniqueNumbers);

[ 1, 2, 3, 4, 5 ]

8. Explain the copyWithin method with an example.

The copyWithin method shallow copies part of an array to another location in the same array, modifying it in place.

let arr = [1, 2, 3, 4, 5];
arr.copyWithin(0, 3); 
console.log(arr); 

[ 4, 5, 3, 4, 5 ]

9. How can you convert an array to a string?

An array can be converted to a string using the toString or join method.

let fruits = ['apple', 'banana', 'cherry'];
console.log(fruits.toString()); 
console.log(fruits.join(', ')); 

apple,banana,cherry
apple, banana, cherry

10. What are array methods keys(), values(), and entries()?

The keys(), values(), and entries() methods return new Array Iterator objects. keys() returns the keys (indexes), values() returns the values, and entries() returns key-value pairs.

let arr = ['a', 'b', 'c'];
let keys = arr.keys();
let values = arr.values();
let entries = arr.entries();


console.log([...keys]); 
console.log([...values]); 
console.log([...entries]);

[ 0, 1, 2 ]
[ 'a', 'b', 'c' ]
[ [ 0, 'a' ], [ 1, 'b' ], [ 2, 'c' ] ]

11. How do you flatten an array of arrays?

You can flatten an array of arrays using the flat method. By default, flat flattens one level deep, but you can specify the depth as an argument.

let nestedArray = [1, [2, [3, [4, 5]]]];
let flatArray = nestedArray.flat(Infinity);
console.log(flatArray); 

[ 1, 2, 3, 4, 5 ]

12. What is the Array.of method?

The Array.of method creates a new Array instance with a variable number of arguments, regardless of number or type of the arguments.

let arr1 = Array.of(1, 2, 3);
console.log(arr1);


let arr2 = Array.of('a', 'b', 'c');
console.log(arr2); 

[ 1, 2, 3 ]
[ 'a', 'b', 'c' ]

13. How do you fill an array with a specific value?

You can fill an array with a specific value using the fill method, which modifies the array in place.

let arr = new Array(5).fill(0);
console.log(arr);

[ 0, 0, 0, 0, 0 ]

14. Explain how to use the Array.prototype.reduceRight method.

The reduceRight method applies a function against an accumulator and each value of the array (from right-to-left) to reduce it to a single value.

let arr = [1, 2, 3, 4];
let result = arr.reduceRight((accumulator, currentValue) => accumulator + currentValue, 0);
console.log(result);

10

15. How can you sort an array of objects by multiple properties?

To sort an array of objects by multiple properties, you can use the sort method with a compare function that first compares one property and, if they are equal, compares another property.

let users = [
  { name: 'John', age: 30 },
  { name: 'Jane', age: 25 },
  { name: 'John', age: 20 }
];
users.sort((a, b) => {
  if (a.name === b.name) {
    return a.age - b.age; 
  } else {
    return a.name.localeCompare(b.name);
  }
});
console.log(users);

[
  { name: 'Jane', age: 25 },
  { name: 'John', age: 20 },
  { name: 'John', age: 30 }
]

16. What is a jagged array and how do you create one in JavaScript?

A jagged array (also known as an "array of arrays") is an array whose elements are arrays of varying lengths.

let jaggedArray = [
  [1, 2, 3],
  [4, 5],
  [6, 7, 8, 9]
];
console.log(jaggedArray);

[ [ 1, 2, 3 ], [ 4, 5 ], [ 6, 7, 8, 9 ] ]

17. How do you implement a binary search in an array?

A binary search can be implemented on a sorted array to efficiently find the index of a specific element. The algorithm repeatedly divides the search interval in half until the value is found or the interval is empty.

function binarySearch(arr, target) {
    let left = 0;
    let right = arr.length - 1;
    while (left <= right) {
        let mid = Math.floor((left + right) / 2);
        if (arr[mid] === target) {
            return mid;
        } else if (arr[mid] < target) {
            left = mid + 1;
        } else {
            right = mid - 1;
        }
    }
    return -1;
}


let sortedArray = [1, 2, 3, 4, 5, 6, 7, 8, 9];
console.log(binarySearch(sortedArray, 5));

4

18. What is the findIndex method and how is it used?

The findIndex method returns the index of the first element in the array that satisfies the provided testing function. If no elements satisfy the testing function, -1 is returned.

let numbers = [1, 2, 3, 4, 5];
let index = numbers.findIndex(num => num > 3);
console.log(index);

3

19. How do you remove falsy values from an array?

Falsy values (false, 0, '', null, undefined, NaN) can be removed from an array using the filter method with a Boolean constructor as the callback.

let mixedArray = [0, 1, false, 2, '', 3, null, 'a', undefined];
let truthyArray = mixedArray.filter(Boolean);
console.log(truthyArray); 

[ 1, 2, 3, 'a' ]

20. Explain the Array.prototype.sort method and its limitations.

The sort method sorts the elements of an array in place and returns the sorted array. By default, it sorts elements as strings in ascending order. This can lead to incorrect results when sorting numbers, so a compare function should be provided for numeric sorting.

let numbers = [10, 2, 30, 4];
numbers.sort(); // Incorrect sorting for numbers
console.log(numbers); 


// Correct numeric sorting
numbers.sort((a, b) => a - b);
console.log(numbers);

[ 10, 2, 30, 4 ]
[ 2, 4, 10, 30 ]

By understanding and practicing these questions and answers, you'll be well-prepared for a JavaScript array-related interview. Remember to explain your thought process clearly and back up your answers with examples whenever possible.