What is the purpose of the parameter to the function inside the then function while using async in javascript?

user_14811913 0
Viewed 130
async function testAsync() {
  return 'Hello Adam!.';
}
 
const any = testAsync();
 
console.log(any.then((msg) => {console.log(msg)}));

I am a complete beginner to JavaScript and trying to understand how async function works. I just don't understand the purpose of msg parameter and how logging that prints the return value.

javascript
5 Answers

I would recommend you to take a look at Promise in JavaScript

Async function returns a promise object.

Every promise object has a 'then' function. The 'then' method accepts a function as it's parameter.

This function will be called when the code inside async function is executed completely.

The parameter inside this function is just a normal function parameter. This function can have multiple parameters depending upon the result returned by async function

In your example, the parameter 'msg' will have the value returned by the function testAsync

user_120299950

Under the hood async functions are wrapped in a Promise, so your async function will be equal to this:

function testAsync() {
  return new Promise((resolve, reject) => {
    resolve("Hello Adam!.");
  });
}

Your parameter comes from the returned promises resolve callback.

user_132360050

The then method returns a promise object. It takes two arguments

p.then(onFulfilled, onRejected);

p.then(function(value) {
   // fulfillment
  }, function(reason) {
  // rejection
});

  • onFullfiled will be called when the promise succeded. It has one argument the return value of the called async function

  • onRejected will be called when the promise fails. It has one argument the reason for the fail

In your example the call succeded and the "msg" param is the return value of your function so "Hello Adam"

user_147470390
  1. async functions return a Promise.
  2. Promises have a resolve and reject pattern
  3. Your msg is the label you give to the resolve output value. Using then means it will only be called once the promise is resolved (i.e once the resolve value is known).

In your example, the 'Hello Adam' value is ~immediately passed out as the resolve value. Another example might be to include a timeout function (to simulate a delayed response):

async function testAsync() {

  return new Promise((resolve) => {
      setTimeout(function () {
          resolve('Hello Adam!')
      }, 1000);
  });
}
 
const any = testAsync();
console.log('testing')
console.log(any.then((msg)=>{console.log(msg)}));

user_14618500

then() is a promise callback when the promise resolves successfully. The data returned by promise will be bonded to the alias you have given in then() signature. For example,

async function testAsync() {
  return 'Hello Adam!.';
}

console.log(testAsync().then((msg)=>{console.log(msg)}));

Here, When you call testAsync function, It resolves and returns a string 'Hello Adam!.', This string can be access as msg (It's just aliased variable, you can name it anything) variable.

user_82033570
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