TS1055
Type '{0}' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
When targeting ES3 or ES5, async functions which try to declare a non-promise return type will throw this error:
class A {
async method(): number {
return 1;
}
}Fix: Use a promise return type.
Change the return type to be a promise:
class A {
async method(): Promise<number> {
return 1;
}
}