CODEWITHSUNDEEP
javascriptsymbols
user15889308
user15889308

Reputation:

What is `async*`?

This example on MDN explains Symbol.asyncIterator.

 const myAsyncIterable = {
    async* [Symbol.asyncIterator]() {
        yield "hello";
        yield "async";
        yield "iteration!";
    }
};

(async () => {
    for await (const x of myAsyncIterable) {
        console.log(x);
        // expected output:
        //    "hello"
        //    "async"
        //    "iteration!"
    }
})();

My problem is what does async* mean in the code?

Upvotes: 0

Views: 128

Answers (1)

Heretic Monkey
Heretic Monkey

Reputation: 12113

The example given makes use of shorthand for creating methods in object literals or classes. Consider the following object definition:

const iHaveAMethod = {
  myMethod() {
    return "cool!";
  }
};
console.log(iHaveAMethod.myMethod()); // cool!

It also uses bracket syntax to define a method using a symbol.

const iAmASymbol = Symbol("neat");
const iHaveAMethod = {
  [iAmASymbol]() {
    return "cool!";
  }
};
console.log(iHaveAMethod[iAmASymbol]()); // cool!

It creates a method on an object that is a generator (using the * syntax).

const iHaveAMethod = {
  * myMethod() {
    yield "cool!";
  }
};
console.log(iHaveAMethod.myMethod().next().value); // cool!

Finally, it's an asynchronous method, and so is marked with async.

const iHaveAMethod = {
  async* myMethod() {
    yield Promise.resolve("cool!");
  }
};
iHaveAMethod.myMethod().next().then(val => console.log(val.value)); // cool!

Combine it with the bracket syntax, and you get async*:

const iAmASymbol = Symbol("neat");
const iHaveAMethod = {
  async* [iAmASymbol]() {
    yield Promise.resolve("cool!");
  }
};
iHaveAMethod[iAmASymbol]().next().then(val => console.log(val.value)); // cool!

That explains how the async* got on there. Async iterators and why you'd want an async generator is a whole different ball of wax.

Upvotes: 2

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