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Implement a minimal Promise/A+ (MyPromise) with then/catch

Q: Implement a minimal Promise/A+ (MyPromise) with then/catch

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Hard > **How to read this doc:** Promises/A+ is a small spec (a few rules about `then` plus one resolution procedure) and it has an official test suite, so this solution was not just demoed: it was run against `promises-aplus-tests` version 2.1.2 and passed all 872 tests. Then the design was stress-tested by removing ONE feature at a time and re-running the suite (each removal fails between 2 and 567 tests), and compared with the native `Promise`, which showed one real difference: how many microtask ticks things take. ## 1. The problem, restated Build a `MyPromise` class: a constructor taking an `executor(resolve, reject)`, a `then(onFulfilled, onRejected)` that returns a NEW promise (that is what makes chaining work), a `catch`, and static `resolve` and `reject`. It must follow Promises/A+: a promise settles once, callbacks always run asynchronously, and returning a promise (or any

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The problem

Problem, as an interviewer would state it: "Implement a MyPromise class with a constructor that takes an executor, then, catch, and static resolve and reject. It must handle chaining and thenables. Discuss edge cases, runtime, and alternatives."

Examples:

Code

MyPromise.resolve(1).then((v) => v + 1).then((v) => { throw new Error("boom " + v); })
  .catch((e) => e.message).then((v) => v + "!");                      // "boom 2!"
MyPromise.resolve({ then(resolve) { resolve("adopted"); } });          // a thenable is adopted: "adopted"

Clarifying questions expected:

Which specification should it follow (Promises/A+), and how asynchronous must the callbacks be?
Should it accept any thenable (an object with a then method), not just its own instances?
Which extras are wanted: finally, all, race, unhandled-rejection reporting?

Code / implementation expected: Yes — a real, working class, ideally run against the official Promises/A+ conformance suite.

promise
custom-promise
thenable

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Solution

Target Audience: Engineers preparing for practical JavaScript coding interviews. Difficulty: Hard

How to read this doc: Promises/A+ is a small spec (a few rules about then plus one resolution procedure) and it has an official test suite, so this solution was not just demoed: it was run against promises-aplus-tests version 2.1.2 and passed all 872 tests. Then the design was stress-tested by removing ONE feature at a time and re-running the suite (each removal fails between 2 and 567 tests), and compared with the native Promise, which showed one real difference: how many microtask ticks things take.

1. The problem, restated

Build a MyPromise class: a constructor taking an executor(resolve, reject), a then(onFulfilled, onRejected) that returns a NEW promise (that is what makes chaining work), a catch, and static resolve and reject. It must follow Promis

The solution is hidden so you can try first

10 min read · 9 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

JavaScriptReal, direct proof: MyPromise matches the native Promise on chaining, thenables, once-only guards and finally, and differs only in microtask tick counts

Practice

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