Interview Q&A / JavaScript

1. Are strings mutable in JavaScript?

   No. Strings are immutable primitives. Methods like toUpperCase, slice and replace never change the original; they return a new string, so you must assign or use the result.

   BeginnerQuick answer1 min read
2. How would you mask all but the last four digits of a card number in JavaScript?

   Take the last four characters with slice(-4) and pad the front back to the original length with padStart(length, '*'). padStart and padEnd fill a string to a target length from the left or right.

   BeginnerQuick answer1 min read
3. What are template literals and why are they better than string concatenation?

   Template literals use backticks. They allow ${expression} interpolation and multi-line strings without escape characters, which makes strings with variables far easier to read than chains of + signs.

   BeginnerQuick answer1 min read
4. What is the difference between replace and replaceAll on strings?

   replace with a string pattern changes only the first match. replaceAll changes every match. With a regular expression, replace needs the g flag to change all matches, and replaceAll throws if the regex lacks it.

   BeginnerQuick answer1 min read
5. What is the difference between slice and substring on a string?

   Both return part of a string without changing it. slice accepts negative indexes, counting from the end, and returns an empty string if start is after end. substring treats negatives as 0 and swaps its arguments if start is greater than end.

   BeginnerQuick answer1 min read
6. Write a function that returns the first non-repeating character in a string.

   Count how many times each character appears using a Map in one pass, then scan the string a second time and return the first character whose count is 1. Return null if none exists.

   BeginnerQuick answerCoding1 min read
7. Write a function to reverse a string and use it to check for a palindrome.

   Split the string into characters, reverse the array and join it back. A palindrome reads the same both ways, so clean the text (lowercase, letters and digits only) and compare it with its reverse.

   BeginnerQuick answerCoding1 min read