typescript/promise-function-async Restriction

🚧 An auto-fix is still under development.

What it does

This rule requires any function or method that returns a Promise to be marked as async.

Why is this bad?

Functions that return Promises should typically be marked as async to make their asynchronous nature clear and to enable the use of await within them. This makes the code more readable and helps prevent common mistakes with Promise handling.

Examples

Examples of incorrect code for this rule:

// Function returning Promise without async
function fetchData(): Promise<string> {
  return fetch("/api/data").then(res => res.text());
}

// Method returning Promise without async
class DataService {
  getData(): Promise<any> {
    return fetch("/api/data").then(res => res.json());
  }
}

// Arrow function returning Promise without async
const processData = (): Promise<void> => {
  return Promise.resolve();
};

Examples of correct code for this rule:

// Async function
async function fetchData(): Promise<string> {
  const response = await fetch("/api/data");
  return response.text();
}

// Async method
class DataService {
  async getData(): Promise<any> {
    const response = await fetch("/api/data");
    return response.json();
  }
}

// Async arrow function
const processData = async (): Promise<void> => {
  await someAsyncOperation();
};

// Functions that don't return Promise are fine
function syncFunction(): string {
  return "hello";
}

// Functions returning Promise-like but not actual Promise
function createThenable(): { then: Function } {
  return { then: () => {} };
}

How to use

To enable this rule in the CLI or using the config file, you can use:

CLI

oxlint --deny typescript/promise-function-async

Config (.oxlintrc.json)

{
  "rules": {
    "typescript/promise-function-async": "error"
  }
}

References

• Rule Source