Interview Q&A / JavaScript

1. What is the difference between debounce and throttle? Implement both.

   Debounce waits until calls stop for a set time and then runs the function once, like search-as-you-type. Throttle runs the function at most once per time window no matter how often it is called, like handling scroll events. Both are built with a closure that holds a timer or a timestamp.

   IntermediateIn-depthCoding3 min read
2. 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
3. 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
4. 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
5. 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
6. Write a function that deep clones an object, including nested arrays and circular references.

   Return primitives as they are. For objects and arrays, create an empty copy, record it in a WeakMap so circular references reuse it, then recursively clone each property. In modern runtimes, structuredClone does this for you.

   IntermediateQuick answerCoding1 min read
7. Write a function that flattens a nested array of any depth.

   Loop through the array; if an item is an array, flatten it recursively and add its items, otherwise add the item. The built-in shortcut is arr.flat(Infinity).

   IntermediateQuick answerCoding1 min read
8. Write a memoize function that caches the results of a pure function.

   Wrap the function and store results in a Map keyed by its arguments. If the key exists, return the cached value; otherwise compute, store and return. It only works safely for pure functions.

   IntermediateQuick answerCoding1 min read
9. Write a retry function with exponential backoff for an async call.

   Loop up to a maximum number of attempts, awaiting the call in a try/catch. On failure, wait delay times 2 to the power of the attempt, then try again. After the last failure, throw the last error.

   IntermediateQuick answerCoding1 min read
10. Write your own version of Function.prototype.bind.

    Add a method on Function.prototype that remembers the original function, the chosen this and any preset arguments, and returns a new function that calls the original with apply, joining preset and new arguments.

    IntermediateQuick answerCoding1 min read
11. Write your own version of Promise.all.

    Return a new promise. Keep a results array and a counter. Wrap each input in Promise.resolve, store its value at its index, and resolve when the counter reaches zero. Reject as soon as any input rejects.

    IntermediateQuick answerCoding1 min read