Understanding Promise vs Promise.all() in JavaScript

In JavaScript,
Promises help handle asynchronous operation in clean and structured way. When works with multiple asynchronous tasks, managing them individually can be tricky.

Promise.all() runs a multiple promises in parallel and handle all their results together.

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Promise

A Promise is an object that represents eventual completion or failure of an asynchronous operation.

It can be in one of three states

• Pending – initial state, neither fulfilled nor rejected.
• Fulfilled – the operation completed successfully.
• Rejected – the operation failed.

const promise = new Promise((resolve, reject) => {
    setTimeout(() => {
        resolve("Success!");
        // reject("Error!");
    }, 1000);
});

promise.then(result => console.log(result))
       .catch(error => console.log(error));

//Here, a single asynchronous task is being handled.

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Promise.all() is method that takes an array of promises and returns new promise that resolves when all of the promises resolve, or rejects if any one promise rejects.

Key points:

• Waits for all promises to finish.
• Returns results in the same order as the input promises.
• Fails fast if any promise rejects.

const promise1 = Promise.resolve(1);
const promise2 = new Promise(resolve => setTimeout(() => resolve(2), 1000));
const promise3 = Promise.resolve(3);

Promise.all([promise1, promise2, promise3])
       .then(results => console.log(results)) // [1, 2, 3]
       .catch(error => console.log(error));

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Feature	Promise	Promise.all()
Input	Single async operation	Array of promises
Output	Result of one operation	Array of all results
Rejection	One promise fails, only it fails	Any one promise fails, entire rejects
Use case	Single async task	Multiple async tasks in parallel