Promise in JS

What is promise?

• A Promise is an object used to handle the result of an asynchronous operation.

Example:

let promise = new Promise((resolve, reject) => {

    let success = true;

    if (success) {
        resolve("Data received successfully");
    } else {
        reject("Failed to receive data");
    }

});

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States in Promise:

A Promise has 3 states:

1. Pending → Operation is still running
2. Fulfilled → Operation completed successfully
3. Rejected → Operation failed

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Why do we need Promises?

Before Promises, asynchronous operations were commonly handled using callbacks.

example:

getUser(function(user) {

    getOrders(user, function(orders) {

        getPayment(orders, function(payment) {

            console.log(payment);

        });

    });

});

When many asynchronous operations depend on each other, callbacks can become deeply nested.

This is called Callback Hell.

Promises make the code easier to organize:

getUser()
    .then(user => getOrders(user))
    .then(orders => getPayment(orders))
    .then(payment => console.log(payment))
    .catch(error => console.log(error));

So, one major reason we need Promises is:

Promises make asynchronous code easier to read, manage, and handle errors .

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Uses of Promises:

1. Handling asynchronous operations
2. Handling success and failure
3. Promise chaining
4. Avoiding Callback Hell
5. Working with multiple asynchronous operations

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Promise to handle success:

When the operation is successful, call resolve().

const promise = new Promise((resolve, reject) => {

    let success = true;

    if (success) {
        resolve("Data received successfully");
    }
});

Then we handle the successful result using .then():

promise.then(result => {
    console.log(result);
});

Promise
   ↓
resolve()
   ↓
FULFILLED
   ↓
.then()
   ↓
Success result

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Promise to handle error:

When the operation fails, call reject().

const promise = new Promise((resolve, reject) => {

    let success = false;

    if (success) {
        resolve("Data received");
    } else {
        reject("Failed to get data");
    }
});

Then handle the error using .catch():

promise.catch(error => {
    console.log(error);
});

Promise
   ↓
reject()
   ↓
REJECTED
   ↓
.catch()
   ↓
Error

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Handling both success and error:

Usually, we handle both together:

const promise = new Promise((resolve, reject) => {

    let success = true;

    if (success) {
        resolve("Data loaded successfully");
    } else {
        reject("Something went wrong");
    }
});

promise
    .then(result => {
        console.log("Success:", result);
    })
    .catch(error => {
        console.log("Error:", error);
    });

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