Async await basic tutorials and examples

This tutorial explains async and awaits tutorials and examples.

Async functions are used to achieve asynchronous programming.

async functions always return a promise. If the function is not returning the promise, It wraps the result in a promise

How to declare async functions in multiple ways

Async functions are used to do asynchronous functions. It pauses the current execution and runs the execution in a separate queue called the event queue. In Real-time, Async function does call API processing

• async normal functions function are declared with the keyword async. It is a normal function

async function getMessage() {
  return "Welcome";
}

which is equal to

async function getMessage() { return Promise.resolve(“Welcome”); }

• async function anonymous expressions Async functions are also declared anonymous and have no name. These can be assigned a variable

const variable = async function() {
  // do something
}

calling Async function returns always Promise. You can explicitly return a promise or value that wrapped with a value

console.log(getMessage())

• Async Arrow functions examples

Arrow functions declared assigned to javascript variables. It is declared with either arguments or without arguments Async Arrow function without arguments

const getMessage = async () => {
  console.log("Hello ")
}

Async with single arguments. Single arguments parenthesis are optional

const getMessage = async (msg) => {
  console.log("Hello ",msg)
}
or 
const getMessage = async msg => {
  console.log("Hello ",msg)
}

Multiple arguments to arrow async functions

const getMessage = async (msg1,msg2) => {
  console.log("Hello ",msg)
}

• Async function also declared with static

static myfunc = async () => {
  console.log("static function ")
}

How to Call async functions

Async functions are asynchronous that always return Promise.

Calling the async function always returns Promise.

• using await keyword async functions are called with the await keyword.

await is waiting for a function to finish its execution. await only called in an async function.

async function getMessage() {
  return "Welcome";
}
console.log(getMessage())
async function callMessage() {
  const result = await getMessage();
  console.log(result); // will print "welcome"
}

callMessage()

If you called await in without async functions, It throws below error

async function getMessage() {
  return "Welcome";
}
console.log(getMessage())
 function callMessage() {
  const result = await getMessage();
  console.log(result); // will print "Hello world!"
}

callMessage()

SyntaxError: await is only valid in async functions and the top-level bodies of modules

• How to call async functions in synchronized functions

You have to use then catch block in synchronized funcitons

then() is used catch successfully resolved promises

async function getMessage() {
  return "Welcome";
}
async function callMessage() {
  getMessage()
  .then(data=>console.log("success",data))
  .catch(error=>console.log("error: ",error))
}

callMessage()// Prints success Welcome

• async functions throws an error

For some reason, async functions throws an error, catch block is called with an error catching

async function getMessage() {
  throw "failed";
}
async function callMessage() {
  getMessage()
  .then(data=>console.log(data))
  .catch(error=>console.log("error",error))
}

callMessage()  // error failed

• async functions reject promise, not returning it

In this example, async functions is completed with Promise reject.

Promise is rejecting, but not returning it

async function getMessage() {
 Promise.reject("fail")
}
async function callMessage() {
  getMessage()
  .then(data=>console.log("success",data))
  .catch(error=>console.log("error",error))
}

callMessage()

Above code throws [UnhandledPromiseRejection: This error originated either by throwing inside of an async function without a catch block or by rejecting a promise which was not handled with .catch(). The promise was rejected with the reason “fail”.] { code: ‘ERR_UNHANDLED_REJECTION’

success undefined

node:internal/process/promises:279
            triggerUncaughtException(err, true /* fromPromise */);
            ^

[UnhandledPromiseRejection: This error originated either by throwing inside of an async function without a catch block or by rejecting a promise which was not handled with .catch(). The promise was rejected with the reason "fail".] {
  code: 'ERR_UNHANDLED_REJECTION'
}

It calls a catch block with an undefined result.

• async functions returns rejected promise

catch function call back is called with an error

async function getMessage() {
 return Promise.reject("fail")
}
async function callMessage() {
  getMessage()
  .then(data=>console.log("success",data))
  .catch(error=>console.log("error",error))
}

callMessage();

Output:

error fail