promise-function-async.md

description
Require any function or method that returns a Promise to be marked async.

🛑 This file is source code, not the primary documentation location! 🛑

See https://typescript-eslint.io/rules/promise-function-async for documentation.

Ensures that each function is only capable of:

• returning a rejected promise, or
• throwing an Error object.

In contrast, non-async, Promise-returning functions are technically capable of either. Code that handles the results of those functions will often need to handle both cases, which can get complex. This rule's practice removes a requirement for creating code to handle both cases.

When functions return unions of Promise and non-Promise types implicitly, it is usually a mistake—this rule flags those cases. If it is intentional, make the return type explicitly to allow the rule to pass.

Examples

Examples of code for this rule

❌ Incorrect

const arrowFunctionReturnsPromise = () => Promise.resolve('value');

function functionReturnsPromise() {
  return Promise.resolve('value');
}

function functionReturnsUnionWithPromiseImplicitly(p: boolean) {
  return p ? 'value' : Promise.resolve('value');
}

✅ Correct

const arrowFunctionReturnsPromise = async () => Promise.resolve('value');

async function functionReturnsPromise() {
  return Promise.resolve('value');
}

function functionReturnsUnionWithPromiseExplicitly(
  p: boolean,
): string | Promise<string> {
  return p ? 'value' : Promise.resolve('value');
}

async function functionReturnsUnionWithPromiseImplicitly(p: boolean) {
  return p ? 'value' : Promise.resolve('value');
}