Async Await is generators and promises

Goal: Async-Await ≈ Generators + Promises

Imagine we are given a piece of code like the one below that uses async functions, how can we rewrite it using only promises and generator functions?

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function doTask1() {
    return new Promise((resolve, reject) => {
        setTimeout(() => resolve(1), 100)
    })
}

function doTask2(arg) {
    return new Promise((resolve, reject) => {
        setTimeout(() => resolve(arg+2), 100)
    })
}

function doTask3(arg) {
    return new Promise((resolve, reject) => {
        setTimeout(() => resolve(arg+3), 100)
    })
}

async function init() {
    const res1 = await doTask1();
    console.log(res1);
    
    const res2 = await doTask2(res1);
    console.log(res2);

    const res3 = await doTask3(res2);
    console.log(res3);

    return res3;
}

init(); // 1\n3\n6

It performs three asynchronous tasks, one after the other where each task depends on the completion of the previous task. Finally, it returns the result of the last task.

How can we rewrite it using generators?

Remember Generators

Remember:

• When a generator function is called, its body is not executed right away.
• Instead it returns an iterator-object which adheres to the iterator protocol i.e. it has a next method.
• The only way to execute the body of the generator is by calling the next method on its iterator-object.
• Every time the next method is called, its body is executed until the next yield expression.
• The result of next() is always an object with two properties:
  • value: the yielded value.
  • done: true if the function code has finished, otherwise false
• This next method also accepts an argument.
• Calling it with an argument
  • Makes the argumentthe value of the current yield expression and
  • Resumes the execution till the next yield expression

First Idea: Generators can yield Promises

By now you would be wondering, how do the generator functions help to achieve our goal?

We need to model an asynchronous flow where we have to wait for certain tasks to finish before proceeding ahead. How can we do that?

Well, the most important insight here is that the generator-functions can yield promises too.

• A generator function can yield a promise (for example an async task), and
• its iterator can be controlled to halt for this promise to resolve (or reject), and then
• recursively proceed with the resolved (or rejected) value to ask for the next iteration.

Rewriting the Async Function as a Generator

This pattern of weaving a an iterator with yielded promises allows us to model our requirement like this:

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function doTask1(arg) {
    return new Promise((resolve, reject) => {
        setTimeout(() => resolve(arg), 100)
    })
}

function doTask2(arg) {
    return new Promise((resolve, reject) => {
        setTimeout(() => resolve(arg+2), 100)
    })
}

function doTask3(arg) {
    return new Promise((resolve, reject) => {
        setTimeout(() => resolve(arg+3), 100)
    })
}

function* init(arg) {
    const res1 = yield doTask1(arg);
    console.log(res1);
    
    const res2 = yield doTask2(res1);
    console.log(res2);

    const res3 = yield doTask3(res2);
    console.log(res3);

    return res3;
}

Notice how this generator function resembles our async function!

If you change yield for await is the same code!

A Function Controlling the Execution of the Generator

But this is only half the story. Now we need a way to execute its body.

We need a function waiter that can control the iterator of this generator function to “wait for the fulfillment of the promise yielded on each iteration”. It has to:

1. Halt every time a promise is yielded and
2. Proceeds once the promise resolves (or rejects).

Write the Function Controlling the Execution of the Generator

Write a function waiter(generator, arg) that creates and iterator by calling generator(arg) and returns a function that traverses the iterator but proceeding with an iteration only when the promise returned by the previous call to iterator.next() has been fulfilled. It will be used like this:

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function waiter(genFun, arg) {
   // ... your code here
}

const doIt = waiter(init, 3);
doIt();

So that, when we run it with the generator above, we obtain:

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➜  learning-async-iteration-and-generators git:(main) ✗ node 07-async-await-equal-generators-plus-promises/example.js 
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8

It sounds complicated, but takes only a few lines to implement.

Heres is a solution

See

• Async-Await ≈ Generators + Promises at https://hackernoon.com/ Cha on July 26th 2017
• Solution to this problem
• Repo ULL-MII-SYTWS-2021/async-await-equal-generators-plus-promises (private)