Typescript: Async * iterator closure not compiling

Question

Why does the following example not compile? Basically, the question is how to properly declare a async iterable closure.

class Test {
  async foo() {
    const c = async () => {
    };
    await c();
  }

  async * bar() {
    const c = async * () => {
    };
    yield * c();
  } 
}

The errors are:

...: error TS1109: Expression expected.
...: error TS1005: ';' expected.

My tsconfig.json:

{
  "compilerOptions": {
    "declaration": true,
    "lib": [
      "es2017",
      "dom",
      "esnext.asynciterable"
    ],
    "module": "commonjs",
    "target": "es2015",
    "sourceMap": true,
    "outDir": "out",
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true,
  },
  "include": [
      "src/**/*.ts"
  ]
}

If I change the example to the following, it works. I use a function instead of arrow syntax:

class Test {
  async foo() {
    const c = async () => {
    };
    await c();
  }

  async * bar() {
    const c = async function *() {
    };
    yield * c();
  } 
}

What error does it fail with?

Carcigenicate
– Carcigenicate

2017-11-19 15:05:42 +00:00
Commented Nov 19, 2017 at 15:05 — Carcigenicate
– Carcigenicate, Commented Nov 19, 2017 at 15:05
Could you please show the content of your tsconfig.json?

Fabian Lauer
– Fabian Lauer

2017-11-19 15:41:55 +00:00
Commented Nov 19, 2017 at 15:41 — Fabian Lauer
– Fabian Lauer, Commented Nov 19, 2017 at 15:41

v-andrew · Accepted Answer · 2019-08-25 00:36:51Z

2

Arrow function ()=>{} and regular function has a few differences which are in play here.

For example arrow function has no this binding.

Furthermore, documentation on generators clearly states that you have to use function syntax as it uses this for iteration.

This is why Async Generators are not going to support Arrow function

edited Aug 25, 2019 at 0:36

answered Nov 19, 2017 at 16:21

v-andrew

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Comments

ctaleck · Accepted Answer · 2019-08-25 07:45:17Z

2

There is no support for arrow-function using async-generators yet; i.e. const c = async *()=> {};.

As with Generators, Async Generators can only be function declarations, function expressions, or methods of classes or object literals. Arrow functions cannot be Async Generators.

https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-3.html

edited Aug 25, 2019 at 7:45

ctaleck

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answered Nov 19, 2017 at 16:14

Meirion Hughes

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