JavaScript

Promise Resolve Called Twice: What It Means and How to Fix It

Error reference

Promise Resolve Called Twice

Language

JavaScript

Severity

Low

When it happens

Calling resolve() or reject() more than once inside a Promise constructor is silently ignored.

Potential fixes

Promises model a single future value. For multiple values, use an EventEmitter, RxJS Observable, or an async generator. Ensure that each code path inside the Promise executor calls either resolve or reject exactly once, then returns.

Deep dive

Calling resolve() or reject() more than once inside a Promise constructor is silently ignored. A Promise can only be settled once. This is not an error per se, but a common source of subtle bugs where developers expect multiple values or callbacks from a single promise.

Why it happens

•A callback-based API calls its callback more than once, and each call triggers resolve.
•Both a success path and an error path call resolve instead of one calling reject.
•An event listener inside a Promise constructor fires multiple times.

Potential fixes

Promises model a single future value. For multiple values, use an EventEmitter, RxJS Observable, or an async generator. Ensure that each code path inside the Promise executor calls either resolve or reject exactly once, then returns.

Examples

JavaScript

Code that triggers the error

const p = new Promise((resolve, reject) => {
  resolve("first");
  resolve("second"); // silently ignored
  reject(new Error("oops")); // also silently ignored
});
const val = await p;
console.log(val); // "first"

Error output

first
// No error is thrown. The second resolve and the reject are silently ignored.

Fixed code

// Promise state is immutable once settled.
// If you need multiple values, use an event emitter or async generator.
async function* stream() {
  yield "first";
  yield "second";
}
for await (const val of stream()) {
  console.log(val);
}

Practice in English

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"I hit an error...

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