Async-Await

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Async Await Promises

The async...await syntax in ES6 offers a new way write more readable and scalable code to handle promises. It uses the same features that were already built into JavaScript.

function helloWorld() {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve('Hello World!');
    }, 2000);
  });
}

async function msg() {
  const msg = await helloWorld();
  console.log('Message:', msg);
}

msg(); // Message: Hello World! <-- after 2 seconds

Asynchronous JavaScript function

An asynchronous JavaScript function can be created with the async keyword before the function name, or before () when using the async arrow function. An async function always returns a promise.

function helloWorld() {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve('Hello World!');
    }, 2000);
  });
}

const msg = async function() { //Async Function Expression
  const msg = await helloWorld();
  console.log('Message:', msg);
}

const msg1 = async () => { //Async Arrow Function
  const msg = await helloWorld();
  console.log('Message:', msg);
}

msg(); // Message: Hello World! <-- after 2 seconds
msg1(); // Message: Hello World! <-- after 2 seconds

JavaScript aysnc await operator

The JavaScript async...await syntax in ES6 offers a new way write more readable and scablable code to handle promises. A JavaScript async function can contain statements preceded by an await operator. The operand of await is a promise. At an await expression, the execution of the async function is paused and waits for the operand promise to resolve. The await operator returns the promise’s resolved value. An await operand can only be used inside an async function.

function helloWorld() {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve('Hello World!');
    }, 2000);
  });
}

async function msg() {
  const msg = await helloWorld();
  console.log('Message:', msg);
}

msg(); // Message: Hello World! <-- after 2 seconds

JavaScript async…await advantage

The JavaScript async...await syntax allows multiple promises to be initiated and then resolved for values when required during execution of the program. As an alternate to chaining .then() functions, it offers better maintainablity of the code and a close resemblance synchronous code.

Async Function Error Handling

JavaScript async functions uses try...catch statements for error handling. This method allows shared error handling for synchronous and asynchronous code.

let json = '{ "age": 30 }'; // incomplete data

try {
  let user = JSON.parse(json); // <-- no errors
  alert( user.name ); // no name!
} catch (e) {
  alert( "Invalid JSON data!" );
}

Using async await syntax

Constructing one or more promises or calls without await can allow multiple async functions to execute simultaneously. Through this approach, a program can take advantage of concurrency, and asynchronous actions can be initiated within an async function. Since using the await keyword halts the execution of an async function, each async function can be awaited once its value is required by program logic.

Resolving JavaScript Promises

When using JavaScript async...await, multiple asynchronous operations can run concurrently. If the resolved value is required for each promise initiated, Promise.all() can be used to retrieve the resolved value, avoiding unnecessary blocking.

let promise1 = Promise.resolve(5);
let promise2 = 44;
let promise3 = new Promise(function(resolve, reject) {
  setTimeout(resolve, 100, 'foo');
});

Promise.all([promise1, promise2, promise3]).then(function(values) {
  console.log(values);
});
// expected output: Array [5, 44, "foo"]