How do async iterators work? error TS2504: Type must have a '[Symbol.asyncIterator]()' method that returns an async iterator

Question

I am trying out the "new" async generator/for dance:

async function* it() {
    yield "Hello";
    yield "World";
}

async function test() {
    for await (const x of it()) {
        console.log()
    }
}

I am getting the following error:

error TS2504: Type must have a '[Symbol.asyncIterator]()' method that returns an async iterator.

How can I make work?

What's the right "typesafe" return type for the "it" function?

Borre Mosch · Accepted Answer · 2017-11-11 21:17:19Z

5

Your code is actually fine, but async iterators are currently an ES.next feature, so you'll have to specify this ES version, e.g. by putting the following property in your tsconfig.json:

"target": "esnext"

Transpiling

If you want to transpile to a previous ES version, TypeScript will not handle this for you automatically. Instead, you can use core-js:

First, install core-js: npm install core-js

Then, edit tsconfig.json:

{
  compilerOptions: {
    "lib": [
      "es2015",
      "esnext.asynciterable"
    ],
    "target": "es2015"
    [...]
  },
  [...]
}

Then, import core-js in to your source file:

import 'core-js/shim';

edited Nov 11, 2017 at 21:17

answered Nov 10, 2017 at 10:59

Borre Mosch

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4 Comments

user3612643 Over a year ago

1. Now do I make the iterator typesafe? 2. Can I transpill to older targets?

Borre Mosch Over a year ago

TypeScript should infer the type of the it function automatically. I added information for transpiling to my answer.

user3612643 Over a year ago

I still need a type since I want to generate an interface function*. Like “interface I { queryAll() : ???<string> }.

user3612643 Over a year ago

GOT IT WORKING! Remarks: - it must be npm i core-js (not corejs) - AsyncIterableIterator<T> is the return type of function*

Vad · Accepted Answer · 2018-10-30 21:19:00Z

-2

You need to shim asyncIterator Symbol.

(Symbol as any).asyncIterator = Symbol.asyncIterator || Symbol.for('Symbol.asyncIterator');

answered Oct 30, 2018 at 21:19

Vad

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