JavaScript Engine Simulator

Objective

Implement three functions that test your mastery of JavaScript async patterns — the exact skills frontend interviews assess.

Task 1: Predict Event Loop Output

Write a function predictOutput() that returns an array of strings in the exact order they would be logged:

function predictOutput() {
  // Return the order of console.logs for this code:
  //
  // console.log('A');
  // setTimeout(() => console.log('B'), 0);
  // Promise.resolve()
  //   .then(() => console.log('C'))
  //   .then(() => console.log('D'));
  // queueMicrotask(() => console.log('E'));
  // console.log('F');

  return ['A', '?', '?', '?', '?', '?']; // Replace with correct order
}

Task 2: Implement Promise.all

Write a promiseAll(promises) function that behaves identically to Promise.all:

Resolves with an array of results when all promises fulfill
Results must maintain the same order as the input promises
Rejects immediately when any promise rejects
Handles non-Promise values (wraps them with Promise.resolve)
Handles empty array input (resolves with [])

function promiseAll(promises) {
  // Your implementation here
}

Task 3: Retry with Exponential Backoff

Write a retryWithBackoff(fn, maxRetries, baseDelay) function:

Calls fn() and returns its result if successful
On failure, backs off exponentially: attempt 0 waits ~baseDelay, attempt 1 ~baseDelay * 2, attempt 2 ~baseDelay * 4
maxRetries counts retries after the first attempt, so maxRetries = 3 makes up to 4 calls to fn. When they are all exhausted, rethrow the last error
Jitter is accepted and encouraged — randomising each wait within its backoff window (for example 50-100% of it) is the version you should defend in an interview, and the lesson explains why. A pure baseDelay * 2^attempt wait is also accepted
Must work with both sync and async functions

async function retryWithBackoff(fn, maxRetries = 3, baseDelay = 1000) {
  // Your implementation here
}

Requirements

All three functions must be implemented
Code must be valid JavaScript (ES2022+)
No external libraries allowed
Include brief comments explaining your reasoning for Task 1

What to Submit

Your submission should contain 1 file section in the editor below: a JavaScript file with all 3 functions.

Instructions