JavaScriptIntermediate8 min read2026-03-01
JavaScript Promises from Scratch
Deep dive into Promise states (pending, fulfilled, rejected), chaining, and combinators (all, race, any, allSettled).
Prerequisites
- JavaScript fundamentals
1. Creating a Custom Promise
Understand how the resolve and reject executor functions control Promise state transitions.
javascript
function delay(ms) {
return new Promise((resolve, reject) => {
if (ms < 0) {
reject(new Error("Delay cannot be negative"));
} else {
setTimeout(resolve, ms);
}
});
}
delay(1000).then(() => console.log("1 second passed!"));Best Practices & Architecture Advice
- Always return values inside .then() handlers to keep the promise chain active.
Common Mistakes to Watch Out For
- •The 'Promise constructor anti-pattern': wrapping an existing promise in another new Promise.
Frequently Asked Questions
What happens if a promise rejection is not caught?
The browser or Node.js runtime triggers an unhandledrejection event, which can terminate Node.js processes.
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