Promise.resolve

mediumcodingJavaScriptPromises~25 min

Asked at Google, Meta, Amazon

Overview

Implement promiseResolve(value) that behaves like Promise.resolve(value).

Constraints

If value is a Promise, return it as-is
If value is a thenable (has .then), wrap it
Otherwise, return a resolved promise

Examples

await promiseResolve(42); // 42
await promiseResolve(Promise.resolve('already')); // 'already'

Solution

Reveal solution

function promiseResolve(value) {
  if (value instanceof Promise) return value;
  if (value && typeof value === 'object' && typeof value.then === 'function') {
    return new Promise((resolve, reject) => value.then(resolve, reject));
  }
  return new Promise(resolve => resolve(value));
}

promise-resolve.js

Promise.resolve

mediumcodingJavaScriptPromises

Overview

Implement promiseResolve(value) that behaves like Promise.resolve(value).

Constraints

If value is a Promise, return it as-is
If value is a thenable (has .then), wrap it
Otherwise, return a resolved promise

Examples

await promiseResolve(42); // 42
await promiseResolve(Promise.resolve('already')); // 'already'

Solution

Reveal solution

function promiseResolve(value) {
  if (value instanceof Promise) return value;
  if (value && typeof value === 'object' && typeof value.then === 'function') {
    return new Promise((resolve, reject) => value.then(resolve, reject));
  }
  return new Promise(resolve => resolve(value));
}

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