JavaScript String Interview Questions and Answers

JavaScript (JS) is the world's most popular lightweight programming language, created by Brendan Eich in 1995. It is a must-have skill for web development, mobile apps, web servers, and more. Whether you aim to be a front-end or back-end developer, mastering JavaScript is crucial for landing top jobs in the industry.

Evernote, LinkedIn, Microsoft, Opera, NASA, and Meta among the top companies rely on JavaScript for its powerful features such as DOM manipulation, asynchronous JavaScript, and error handling. To work in these renowned firms you must be excellent at JavaScript interviews.

JavaScript strings are fundamental aspects of the language used frequently to maintain data lists. This article will deal with a complete set of questions required during an interview ranging from basic to advanced. There is a detailed answer coupled with code examples which can help you prepare adequately enough.

Our Top 50 JavaScript String 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 String Basic Interview Questions

1. What is a string in JavaScript?

A string in JavaScript is a sequence of characters used to represent text. Strings are immutable, meaning once created, they cannot be changed. They are enclosed in single quotes (' '), double quotes (" "), or backticks (` `) for template literals.

2. How do you create a string in JavaScript?

You can create a string by enclosing the characters in quotes. For example:

let str1 = 'Hello';
let str2 = "World";
let str3 = `Hello World`;

Strings can be defined using different types of quotes depending on the use case, such as embedding variables using backticks.

3. How do you find the length of a string in JavaScript?

The length property of a string returns the number of characters in the string. For example:

let str = 'Hello';console.log(str.length); 

This property helps in determining the size of the string.

4. How can you concatenate two strings in JavaScript?

You can concatenate strings using the + operator or the concat() method. For example:

let str1 = 'Hello';let str2 = 'World';
let result = str1 + ' ' + str2;
let result2 = str1.concat(' ', str2); 

Concatenation combines multiple strings into one.

5. What is the difference between == and === when comparing strings in JavaScript?

The == operator compares strings for equality after type conversion, while === compares both value and type without type conversion. For example:

console.log('5' == 5);
console.log('5' === 5); 

It's generally recommended to use === for strict equality to avoid unexpected results.

6. How do you convert a string to uppercase or lowercase in JavaScript?

You can use the toUpperCase() and toLowerCase() methods to convert a string to uppercase or lowercase respectively. For example:

let str = 'Hello World';
console.log(str.toUpperCase()); // Output: 'HELLO WORLD'
console.log(str.toLowerCase()); // Output: 'hello world'

These methods are useful for case-insensitive comparisons or formatting.

7. How do you extract a part of a string in JavaScript?

You can use methods like slice(), substring(), and substr() to extract parts of a string. For example:

let str = 'Hello World';
console.log(str.slice(0, 5)); // Output: 'Hello'
console.log(str.substring(0, 5)); // Output: 'Hello'
console.log(str.substr(0, 5)); // Output: 'Hello'

These methods allow you to get a portion of the string based on specified indices.

8. How do you check if a string contains a specific substring in JavaScript?

You can use the includes() method to check if a string contains a specific substring. For example:

let str = 'Hello World';
console.log(str.includes('World')); // Output: true
console.log(str.includes('world')); // Output: false

This method returns a boolean indicating whether the substring is found.

9. How do you replace a substring within a string in JavaScript?

The replace() method is used to replace a substring with another string This method replaces the first occurrence of the specified substring. For example:

let str = 'Hello World';
let newStr = str.replace('World', 'JavaScript'); // Output: 'Hello JavaScript'

10. How do you trim whitespace from a string in JavaScript?

You can use the trim(), trimStart(), and trimEnd() methods to remove whitespace from both ends, the start, or the end of a string. These methods are useful for cleaning up input data. For example:

let str = '  Hello World  ';
console.log(str.trim()); // Output: 'Hello World'
console.log(str.trimStart()); // Output: 'Hello World  '
console.log(str.trimEnd()); // Output: '  Hello World'

JavaScript String Intermediate Interview Questions

1. How do you split a string into an array in JavaScript?

The split() method splits a string into an array of substrings based on a specified separator. This method is helpful for parsing and processing string data. For example:

2. What is the purpose of the charAt() method in JavaScript?

The charAt() method returns the character at a specified index in a string. For example:

let str = 'Hello';
console.log(str.charAt(1)); // Output: 'e'

It’s useful for accessing individual characters in a string.

3. How do you repeat a string multiple times in JavaScript?

The repeat() method repeats a string a specified number of times. This method is useful for generating repeated patterns. For example:

let str = 'Hello';
console.log(str.repeat(3)); // Output: 'HelloHelloHello'

4. How do you pad a string to a certain length in JavaScript?

You can use padStart() and padEnd() methods to pad a string to a certain length. For example:

let str = '5';
console.log(str.padStart(3, '0')); // Output: '005'
console.log(str.padEnd(3, '0')); // Output: '500'

These methods are useful for formatting numbers and strings.

5. What does the localeCompare() method do in JavaScript?

The localeCompare() method compares two strings in the current locale and returns a number indicating whether the string comes before, after, or is the same as the compared string. For example:

let str1 = 'apple';
let str2 = 'banana';
console.log(str1.localeCompare(str2)); // Output: -1

This method is useful for sorting strings alphabetically.

6. How do you find the index of a substring in a string in JavaScript?

The indexOf() method returns the index of the first occurrence of a substring, or -1 if not found. It’s useful for searching within strings. For example:

let str = 'Hello World';
console.log(str.indexOf('World')); // Output: 6
console.log(str.indexOf('world')); // Output: -1

7. What is the difference between slice() and substring()?

Both slice() and substring() extract parts of a string, but slice() allows negative indices and substring() does not. For example:

let str = 'Hello World';
console.log(str.slice(-5)); // Output: 'World'
console.log(str.substring(6)); // Output: 'World'

Choose based on whether you need negative indexing.

8. How do you convert a string to an array of characters in JavaScript?

You can use the split('') method to convert a string to an array of characters. For example:

let str = 'Hello';
let arr = str.split(''); // Output: ['H', 'e', 'l', 'l', 'o']

This method is useful for iterating over each character.

9. How do you check if a string starts with a specific substring in JavaScript?

The startsWith() method checks if a string starts with a specified substring. For example:

let str = 'Hello World';
console.log(str.startsWith('Hello')); // Output: true

It’s useful for validation and filtering.

10. How do you check if a string ends with a specific substring in JavaScript?

The endsWith() method checks if a string ends with a specified substring. For example:

let str = 'Hello World';
console.log(str.endsWith('World')); // Output: true

It’s useful for validation and filtering.

11. How do you use template literals in JavaScript?

Template literals are enclosed in backticks (`) and can contain placeholders for variables and expressions. For example:

let name = 'World';
let greeting = `Hello, ${name}!`; // Output: 'Hello, World!'

They are useful for embedding variables and expressions in strings.

12. How do you escape special characters in a string in JavaScript?

Use a backslash (\) to escape special characters:

let str = 'He said, "Hello World!"';
let escapedStr = 'He said, \"Hello World!\"';

Escaping special characters ensures they are treated as literals.

13. How do you compare two strings in JavaScript?

Use the localeCompare() method for locale-aware comparisons:

let str1 = 'apple';
let str2 = 'banana';
console.log(str1.localeCompare(str2)); // Output: -1

It compares strings according to the sort order of the current locale.

14. What is a template literal and how do you use it?

Template literals are strings enclosed in backticks that allow embedded expressions:

let name = 'World';
let greeting = `Hello, ${name}!`; // Output: 'Hello, World!'

They are useful for multi-line strings and string interpolation.

15. How do you access characters in a string using bracket notation?

You can access characters in a string using bracket notation, similar to arrays:

let str = 'Hello';
console.log(str[1]); // Output: 'e'

This method is useful for iterating over individual characters in a string.

16. How do you convert a string to an array of words in JavaScript?

Use the split() method to convert a string to an array of words:

let str = 'Hello World';
let words = str.split(' '); // Output: ['Hello', 'World']

This method is useful for parsing sentences into words.

17. How do you find the last occurrence of a substring in a string in JavaScript?

Use the lastIndexOf() method:

let str = 'Hello World Hello';
console.log(str.lastIndexOf('Hello')); // Output: 12

This method returns the index of the last occurrence of the substring.

18. How do you check if a string matches a regular expression in JavaScript?

Use the match() method:

let str = 'Hello World';
console.log(str.match(/World/)); // Output: ['World']

This method checks if the string matches the specified regular expression.

19. How do you search for a match in a string using a regular expression in JavaScript?

Use the search() method:

let str = 'Hello World';
console.log(str.search(/World/)); // Output: 6

This method returns the index of the match, or -1 if not found.

20. How do you replace all occurrences of a substring in a string in JavaScript?

Use the replace() method with a global regular expression:

let str = 'Hello World Hello';
let newStr = str.replace(/Hello/g, 'Hi'); // Output: 'Hi World Hi'

This method replaces all occurrences of the specified substring.

JavaScript String Advance Interview Questions

Here are 20 advanced JavaScript string interview questions along with detailed answers and examples. These questions are designed to challenge your understanding of JavaScript string handling and manipulation techniques that are important for complex problem-solving in real-world applications.

1. What is the difference between substring() and substr() in JavaScript?

The substring() method extracts characters between two indices, while substr() extracts a specified number of characters starting from a given index:

let str = 'Hello World';
console.log(str.substring(6, 11)); // Output: 'World'
console.log(str.substr(6, 5)); // Output: 'World'

substring() takes two arguments: the starting and ending indices, and extracts characters from the start index up to, but not including, the end index. If the end index is omitted, it extracts to the end of the string. substr(), on the other hand, takes the start index and the number of characters to extract. While substring() is useful when you know the exact indices, substr() is more useful when you know the length of the substring you need.

2. How do you reverse a string in JavaScript?

You can reverse a string by converting it into an array, reversing the array, and then joining it back into a string. For example:

let str = 'Hello';
let reversedStr = str.split('').reverse().join('');
console.log(reversedStr); // Output: 'olleH'

3. How can you count the occurrence of a specific character in a string?

You can use the split() method to create an array where each instance of the character is an element, and then subtract one from the length of this array. For example, to count the number of 'l's in "Hello":

let str = 'Hello';
let count = str.split('l').length - 1;
console.log(count); // Output: 2

4. What is the normalize() method in JavaScript strings and when should it be used?

The normalize() method returns the Unicode Normalization Form of a given string, which is important when comparing strings with combining characters (accents, diacritics).

5. How do you format a string using replace() and regular expressions?

You can dynamically replace parts of a string using regular expressions with the replace() method. For example, to mask characters in an email:

let email = '[email protected]';
let maskedEmail = email.replace(/(.{2})(.*)(@.+)/, (match, p1, p2, p3) => p1 + '*'.repeat(p2.length) + p3);
console.log(maskedEmail); // Output: 'us***********@example.com'

6. How do you convert a string into a URL slug?

You can create a URL slug by converting the string to lowercase, replacing spaces with hyphens, and encoding URI components:

let title = 'JavaScript Advanced String Handling';
let slug = title.toLowerCase().replace(/\s+/g, '-');
console.log(encodeURI(slug)); // Output: 'javascript-advanced-string-handling'

7. How do you perform case-insensitive string comparison?

Convert both strings to the same case (either upper or lower) before comparison. For example:

let str1 = 'hello';
let str2 = 'HELLO';
let isEqual = str1.toLowerCase() === str2.toLowerCase();
console.log(isEqual); // Output: true

8. How do you extract hash tags from a string?

Use regular expressions to match the hash tag pattern. For example:

let text = 'Loving #JavaScript and #coding';
let hashtags = text.match(/#\w+/g);
console.log(hashtags); // Output: ['#JavaScript', '#coding']

9. How do you count the number of words in a string?

Split the string by spaces and other punctuation, and count the elements of the array:

let sentence = 'Hello world, welcome to the universe.';
let words = sentence.match(/\b\w+\b/g).length;
console.log(words); 
// Output: 6

10. What are tagged template literals and how do you use them?

Tagged template literals allow you to parse template literals through a function. The first argument contains an array of string values, and the rest are the interpolated values. For example:

function highlight(strings, ...values) {
  return strings.reduce((result, str, i) => `${result}${str}<strong>${values[i] || ''}</strong>`, '');
}
let name = 'world';
console.log(highlight`Hello, ${name}!`); // Output: 'Hello, <strong>world</strong>!'

11. how do you remove all non-alphanumeric characters from a string?

Use a regular expression to replace non-alphanumeric characters:

let str = 'Hello, World! 123.';
let cleanedStr = str.replace(/[^a-z0-9]/gi, '');
console.log(cleanedStr); // Output: 'HelloWorld123'

12. How can you encode and decode HTML characters in a string?

Use the encodeURI or encodeURIComponent functions for encoding, and decodeURI or decodeURIComponent for decoding. For example:

let html = '<div class="foo">Bar</div>';
let encodedHtml = encodeURIComponent(html);
console.log(encodedHtml); // Output: '%3Cdiv%20class%3D%22foo%22%3EBar%3C%2Fdiv%3E'
console.log(decodeURIComponent(encodedHtml)); // Output: '<div class="foo">Bar</div>'

13. How do you convert a string to a number and vice versa?

Use the parseInt, parseFloat, or unary + operator to convert a string to a number. Use toString() or template literals to convert a number to a string:

let numStr = '123';
let num = +numStr;  // Convert to number
let str = num.toString();  // Convert back to string
console.log(num, str); // Output: 123 '123'

14. How do you find the longest word in a string?

Split the string into words, and reduce the array to find the longest word:

let sentence = 'JavaScript is awesome';
let longestWord = sentence.split(' ').reduce((longest, currentWord) => {
  return currentWord.length > longest.length ? currentWord : longest;
}, '');
console.log(longestWord); 
// Output: 'JavaScript'

15. How do you format a string to title case?

Split the string into words, capitalize the first letter of each word, and join them back:

let sentence = 'make javascript great again';
let titleCase = sentence.split(' ').map(word => word[0].toUpperCase() + word.substring(1)).join(' ');
console.log(titleCase); // Output: 'Make JavaScript Great Again'

16. How do you convert camelCase to snake_case and vice versa in JavaScript?

To convert from camelCase to snake_case, use regular expressions to insert underscores before uppercase letters, and convert the entire string to lowercase. To convert back, remove underscores, and capitalize the following letters:

let camelCase = 'camelCaseString';
let snake_case = camelCase.replace(/([A-Z])/g, '_$1').toLowerCase();
console.log(snake_case); // Output: 'camel_case_string'

let snakeCase = 'snake_case_string';
let camelCaseString = snakeCase.replace(/(_\w)/g, (m) => m[1].toUpperCase());
console.log(camelCaseString); // Output: 'snakeCaseString'

17. How can you capitalize the first letter of each sentence in a paragraph?

Split the paragraph into sentences, capitalize the first letter of each, and then join them back together:

let paragraph = 'hello world. welcome to javascript.';
let capitalized = paragraph.split('. ').map(sentence => sentence[

18. How do you implement a function to check if a string is a palindrome?

A palindrome is a string that reads the same backward as forward. To check if a string is a palindrome, you can compare the string to its reversed version:

function isPalindrome(str) {
  let cleanedStr = str.replace(/[^A-Za-z0-9]/g, '').toLowerCase();
  let reversedStr = cleanedStr.split('').reverse().join('');
  return cleanedStr === reversedStr;
}

console.log(isPalindrome('A man, a plan, a canal, Panama')); // Output: true
console.log(isPalindrome('Hello')); // Output: false

19. How do you find and highlight all occurrences of a substring in a string?

To find and highlight all occurrences of a substring in a string, you can use the replace method with a global regular expression and a function:

function highlightSubstring(str, substring) {
  let regex = new RegExp(`(${substring})`, 'gi');
  return str.replace(regex, '<mark>$1</mark>');
}

let text = 'JavaScript is great. I love JavaScript!';
let highlightedText = highlightSubstring(text, 'JavaScript');
console.log(highlightedText);
// Output: '<mark>JavaScript</mark> is great. I love <mark>JavaScript</mark>!'

20. How do you convert a string containing a list of comma-separated values into an array, and then filter out empty values?

You can split the string by commas to create an array, and then use the filter method to remove any empty values:

function csvToArray(csvString) {
  let array = csvString.split(',');
  return array.filter(value => value.trim() !== '');
}

let csvString = 'apple, ,banana,,orange, ,';
let array = csvToArray(csvString);
console.log(array); 
// Output: ['apple', 'banana', 'orange']

Strings in JavaScript

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