21. Promise Limit - K Parallel Tasks

Implement promiseLimit(tasks, k) to control the number of concurrently executing asynchronous tasks.

You are given an array of functions, where each function returns a Promise. Execute these tasks such that at most k promises are running at any given time.

Concurrency Control

• At most k tasks should run in parallel.
• When a running task settles (resolves or rejects), the next pending task should start.

Lazy Execution

• Tasks must not be executed until a concurrency slot is available.

Order Preservation

• The final result array must preserve the order of the input tasks, regardless of completion timing.

Rejection Handling

• If any task rejects, immediately reject the returned promise with the same error.
• No further tasks should be started after a rejection (similar to Promise.all).

Example

const tasks = [
  () => new Promise(resolve => setTimeout(() => resolve(1), 100)),
  () => new Promise(resolve => setTimeout(() => resolve(2), 200)),
  () => new Promise(resolve => setTimeout(() => resolve(3), 300)),
  () => new Promise(resolve => setTimeout(() => resolve(4), 400)),
];

promiseLimit(tasks, 2).then(results => {
  console.log(results); // [1, 2, 3, 4]
});