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. 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
3. 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
4. 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
5. 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
6. 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
7. 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