Interview Q&A / JavaScript

1. How do you remove duplicate values from an array?

   The shortest way is [...new Set(arr)], because a Set stores each value once and keeps insertion order. Filtering with indexOf also works but is slower. For objects, dedupe by a key using a Map.

   BeginnerQuick answerCoding1 min read
2. Write a function once(fn) that lets fn run only the first time it is called.

   Return a wrapper that keeps a called flag and the first result in a closure. On the first call, run fn and store the result; on later calls, return the stored result without running fn.

   BeginnerQuick answerCoding1 min read
3. 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
4. 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