promise-function-async

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

🔧

Some problems reported by this rule are automatically fixable by the --fix ESLint command line option.

💭

This rule requires type information to run.

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.

.eslintrc.cjs

module.exports = {
  "rules": {
    "@typescript-eslint/promise-function-async": "error"
  }
};

Try this rule in the playground ↗

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');
}

// An explicit return type that is not Promise means this function cannot be made async, so it is ignored by the rule
function functionReturnsUnionWithPromiseExplicitly(
  p: boolean,
): string | Promise<string> {
  return p ? 'value' : Promise.resolve('value');
}

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

Options

This rule accepts an options object with the following properties:

interface Options {
  /**
   * Whether to consider `any` and `unknown` to be Promises.
   */
  allowAny?: boolean;
  /**
   * Any extra names of classes or interfaces to be considered Promises.
   */
  allowedPromiseNames?: string[];
  checkArrowFunctions?: boolean;
  checkFunctionDeclarations?: boolean;
  checkFunctionExpressions?: boolean;
  checkMethodDeclarations?: boolean;
}

const defaultOptions: Options = [
  {
    allowAny: true,
    allowedPromiseNames: [],
    checkArrowFunctions: true,
    checkFunctionDeclarations: true,
    checkFunctionExpressions: true,
    checkMethodDeclarations: true,
  },
];

Resources

• Rule source
• Test source