2637. Promise Time Limit

Problem

Given an asynchronous function fn and a time t in milliseconds, return a new time limited version of the input function. fn takes arguments provided to the time limited function.

The time limited function should follow these rules:

• If the fn completes within the time limit of t milliseconds, the time limited function should resolve with the result.
• If the execution of the fn exceeds the time limit, the time limited function should reject with the string "Time Limit Exceeded".

Example 1:

Input: fn = async (n) => { await new Promise(res => setTimeout(res, 100)); return n * n; },
       inputs = [5], t = 50
Output: { rejected: "Time Limit Exceeded", time: 50 }

Solution

Promise.race resolves or rejects with whichever promise settles first. So we race the real work against a “timer” promise that rejects after t milliseconds: if fn finishes first, we get its result; if the timer fires first, we get the rejection.

var timeLimit = function (fn, t) {
  return async function (...args) {
    return Promise.race([
      fn(...args),
      new Promise((_, reject) =>
        setTimeout(() => reject("Time Limit Exceeded"), t),
      ),
    ]);
  };
};

Complexity Analysis

• The wrapper only adds one extra promise and one timer per call

Notes

• Promise.race does not cancel the losing promise: if the timer wins, fn keeps running in the background; its result is just ignored. Real cancellation would need something like AbortController.
• The timer promise never resolves, it only rejects, so it can never accidentally “win” with a value.
• Classic pattern for wrapping APIs that may hang: timeouts on fetch calls, database queries, etc.