How To Await Multiple Promises

As a software engineer, working with asynchronous code is essential. When dealing with multiple asynchronous tasks, using promises can help manage the flow of your code efficiently. In this article, we'll dive into the concept of awaiting multiple promises in JavaScript and how you can effectively handle them in your projects.

When you have several asynchronous operations that need to be executed concurrently and you want to wait for all of them to complete, the Promise.all() method comes to the rescue. This method takes an array of promises as its argument and returns a single promise that resolves when all of the input promises have resolved.

Here's a simple example to illustrate how you can use Promise.all():

Javascript

const promise1 = new Promise(resolve =&gt; setTimeout(resolve, 1000, 'First promise'));
const promise2 = new Promise(resolve =&gt; setTimeout(resolve, 2000, 'Second promise'));

Promise.all([promise1, promise2]).then(values =&gt; {
    console.log(values);
});

In this example, we create two promises with setTimeout functions simulating asynchronous tasks that resolve after 1 and 2 seconds, respectively. By using Promise.all(), we can wait for both promises to resolve and then log their values once they are all done.

What if you want to handle multiple promises sequentially instead of in parallel? Enter the async/await syntax. With async functions and the await keyword, you can execute promises in a synchronous manner within an asynchronous context.

Here's an example demonstrating how to await multiple promises sequentially using async/await:

Javascript

const delay = ms =&gt; new Promise(resolve =&gt; setTimeout(resolve, ms));

async function sequentialPromises() {
    await delay(1000);
    console.log('First promise resolved');
    
    await delay(2000);
    console.log('Second promise resolved');
}

sequentialPromises();

In this snippet, the sequentialPromises function uses the async keyword to define an asynchronous function. Inside this function, we use the await keyword before each promise to wait for it to resolve before continuing to the next one. This way, the promises are executed sequentially.

However, sometimes you may need to handle a mix of parallel and sequential promises. In such cases, you can combine Promise.all() with async/await to achieve the desired behavior.

Here's a code snippet that combines both approaches to handle a mix of parallel and sequential promises:

Javascript

async function mixedPromises() {
    const result1 = await Promise.resolve('First parallel promise');
    
    const [result2, result3] = await Promise.all([
        delay(1000).then(() =&gt; 'Sequential promise 1'),
        delay(2000).then(() =&gt; 'Sequential promise 2')
    ]);
    
    console.log(result1);
    console.log(result2);
    console.log(result3);
}

mixedPromises();

In this example, we start by directly awaiting a parallel promise using the await keyword. Then, we define an array of promises inside Promise.all() to run two sequential promises concurrently. Finally, we log the results in the desired order to handle both types of promises effectively.

By mastering the techniques presented in this article, you can confidently manage multiple promises in your JavaScript applications to streamline asynchronous operations and improve the overall performance of your code. Remember to practice and experiment with these concepts to become more proficient in handling asynchronous tasks with promises. Happy coding!

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