Async/Await

2020-05-25   2 min read

This is the latter half of promise & async/await revising session post.

Contents

  • Definitions
  • Syntax
  • Switch to async/await from promises

(1) Definitions

MDN says

"An asynchronous function is a function which operates asynchronously via the event loop, using an implicit promise to return its result. [...]"

Another important thing to remember:

An async function can contain an await expression that PAUSES the execusion of the async function and WAITS for the passed promise's resolution, and then resumes the asyncfunction's execusion and returns the resolved value. Remember, the await keywords is only valid inside async functions.

Async/await was created to simplify the process of working with and writing chained promises. Hence, async functions return a promise. If the function throws an error, the Promise will be rejected. If the promise returns a value, the promise will be resolved.

(2) Syntax

async function doSomething() {}

// arrow function

const doSomething = async() => {}

(3) Switch to async/await from promises

const doubleAfter2seconds = x => {
   return new Promise(resolve => {
      setTimeout(() => {
         resolve(x * 2)
      }, 4000)
   });
}

doubleAfter2seconds(10).then(value => {
   console.log(value)
});

If I was to add up the resolved value, I'd do

const addPromise = x => {
   return new Promise(resolve => {
      doubleAfter2seconds(10).then((a) => {
         doubleAfter2seconds(20).then((b) => {
            doubleAfter2seconds(30).then(c) => {
               resolve(x + a + b + c)
            }
         })
      })
   })
}

addPromise(10).then((sum) => {
   console.log(sum)
})

which is...not the easiest thing to read. If I use async/await, this above become as simple as below

const addPromiseAsync = async(x) => {
   const a = await doubleAfter2seconds(10);
   const b = await doubleAfter2seconds(20);
   const c = await doubleAfter2seconds(30);
   return x + a + b + c;
}

addPromiseAsync(10).then((sum) => {
   console.log(sum);
})

Thanks to await keyword, the function addPromiseAsync() pauses for 2 seconds while it waits for the promise to resolve. Once the promise resolves, variable a, b, c will have resolved values respectively.


Typos/comments/suggestions are welcomed on GitHub issues

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