async-function-assignability

:mag: requires type information

Diallows assinging a Promise-returning function to a void-returning type.

Rationale

TypeScript allows functions with any return type to be assigned to a function type with a void return type. That's especially a problem with Promise-returning functions as the caller of the function doesn't expect a returned value and therefore doesn't await the Promise. Depending on the runtime there might be an error if a rejected promise is not handled or there may be some hard to debug race condition.

Examples

:thumbsdown: Examples of incorrect code

// these functions don't care about the return value of the callbacks
[1, 2, 3].forEach(async (v) => doStuff(v));
setTimeout(async () => doStuff(), 100);

class C {
  foo() {}
}
class D extends C {
  async foo() {} // overriding 'void'-returning method with 'Promise'-returning method
}

Related Rules

• await-async-result
• await-only-promise
• no-return-await
• try-catch-return-await