ES6 Callbacks and Async Operation

Callbacks and Async Operation

In Javascript the code is executed line-by-line in a sequence so when we run a parallel operation or asynchronous operation like fetching data from backend, javascript doesn't waits for the response it simply executes the next line of code. We give the asynchronous operation a function to call when it is completed. This function is called a Callback Function.


await - Asynchronous actions are the action that we initiate now  & they finish later.


Eg - SetTimeout


Synchronous actions are the actions that initiate & finish one-by-one.


Async - Asynchronous it allows a program to run a function without stop/hold/freezing the entire program. This is done using the Async/Await keyword.

Async/Await make it easier to write promises.

The keyword 'async' before a function makes the function return a promises.

The keyword await is used inside async keyword.

Introduction to Promises

A promises is used to handle the asynchronous result of an operation. It defers the execute of a code block until an asynchronous request is completed. This way, other operations can keep running without interruption.


A promises has 3 states :

# Pending - It means the operation is going on.

# Fulfilled - It means the operation was completed.

# Rejection - It means the operation did not completed and an error can be thrown.

<script>

async function atul() {
      let promises = new promise((resolve, reject) => {
      setTimeout(() =>
                    resolve("Done"), 3000)
      
      });

let result = await promise;
    alert(Result);
    console.log("The result is ", result);
    
    }

atul();

</script>

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