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[]>:

What am I missing?

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[]>:

What am I missing?

• javascript
• typescript
• promise

Share Improve this question Follow edited Mar 19, 2019 at 7:02 Eliya Cohen asked Mar 19, 2019 at 6:56 Eliya CohenEliya Cohen 11.5k1414 gold badges7676 silver badges124124 bronze badges 4

• 1 I assume your validate call should be a call to to, right? – ShamPooSham Commented Mar 19, 2019 at 7:00
• @ShamPooSham you're right. My mistake. I have updated my question – Eliya Cohen Commented Mar 19, 2019 at 7:02
• Can you put together an small example where this happens? – distante Commented Mar 19, 2019 at 7:07
• 1 I got exactly the same error in pure Javascript due to a branch in the called function which mistakenly returned a single element instead of an array. – Craig Hicks Commented Aug 11, 2019 at 15:53

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1 Answer 1

Sorted by: Reset to default 4

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]);
}