CODEWITHSUNDEEP
javascripttypescriptpromise
Eliya Cohen
Eliya Cohen

Reputation: 11508

Typescript - "... is not a function or its return value is not iterable"

I have this helper function:

export function to(promise: Promise<any>) {
  return promise
    .then((data: any) => [null, data])
    .catch((err: Error) => [err, null]);
}

This function (in theory) should help me catch errors while using await in functions. for example:

const [err, data] = await to(validate(card));

The problem is that on runtime, I get the following error:

to is not a function or its return value is not iterable

While the expected return signature should be Promise<[Error, null]> Promise<[null, Error]>, it looks like returns (again, in theory, because it actually fails): Promise<any[] | Error[]>:

function to

What am I missing?

Upvotes: 4

Views: 13247

Answers (1)

ttulka
ttulka

Reputation: 10892

I think the problem is caused by duck typing, TypeScript is not able to guess the return type correctly from the expressions.

You can type it explicit:

function to(promise: Promise<any>): Promise<[Error, any]> {
    return promise
        .then((data: any) => [null, data] as [Error, any])
        .catch((err: Error) => [err, null] as [Error, any]);
}

Upvotes: 4

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