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Async-Await

Making promises easier since 2017

Async

async is a keyword used with functions. It makes it return a promise.

I grabbed a function from the Promises notes that I had written before :

function funct()
{
  return Promise.resolve("Hello");
}

Now if we re-write this using async keyword :

async function funct()
{
    return "Hello";
}

Let’s try using this in the isEven() program.

First, we define an async function and throw the ‘defining a random number’ part inside it. We then use a ternary operator to check if our number would be even or not, and by that way, we create a Promise!

async function isEven()
{
  let n = Math.floor(Math.random() * 100);
  console.log("Number: " + n);
  //always resolving part
  return (n % 2 === 0) ? 1 : 0;
}

The return part is always resolving, as in it doesn’t return 0 in the first place. We need it to make it throw an error for odd numbers :

async function isEven()
{
  let n = Math.floor(Math.random() * 100);
  console.log("Number: " + n);
  if (n % 2 === 0)
    return 1; //resolve
  else
    throw "Odd"; //reject
}

Now we modify the 2nd part of it :

isEven().then(
    (value) =>
    {
        console.log("This is even, see: " + value);
    },
    (error) =>
    {
        console.log("This is odd, see: " + error);
    }
);

Here is the final program :

async function isEven()
{
  let n = Math.floor(Math.random() * 100);
  console.log("Number: " + n);
  if (n % 2 === 0)
    return 1; //resolve
  else
    throw "Odd"; //reject
}

isEven().then(
    (value) =>
    {
        console.log("This is even, see: " + value);
    },
    (error) =>
    {
        console.log("This is odd, see: " + error);
    }
);

Await

await is another keyword that has co-dependency issues with async. Simply put, await can only be used with an async function. Any other function and it’ll run for the hills.

let someVar = await asyncFunction();

It makes the function pause the execution, chill around for a while and continue after the async function returns a Promise object.

It also means that you don’t need to chain a hundred functions in .then().

We see this in our previous isEven() function’s previous version :

async function isEven()
{
  let n = Math.floor(Math.random() * 100);
  console.log("Number: " + n);
  return (n % 2 === 0) ? 1 : 0;
}

Now we add another function which waits for isEven() to finish executing and then gives the final judgement :

async function check()
{
  let final = await isEven();
  if (final===1)
    console.log("Even");
  else
    console.log("Odd");
}
check();

So our entire program barely takes a few line [even less if simplified]!

async function isEven()
{
  let n = Math.floor(Math.random() * 100);
  console.log("Number: " + n);
  return (n % 2 === 0) ? 1 : 0;
}

async function check()
{
  let final = await isEven();
  (final===1) ? console.log("Even") : console.log("Odd");
}
check();

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