What is Promise.all, and when to use it?

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Aryan Kumar · Answer

Promise.all is a built-in method in JavaScript that is used to handle multiple Promises simultaneously. It takes an iterable (such as an array or any iterable object) of Promises as input and returns a new Promise. This new Promise resolves when all the input Promises have successfully resolved, or it rejects immediately if any of the input Promises are rejected.
Here's the basic syntax of Promise.all:
Promise.all(iterable);
iterable: An iterable object (usually an array) containing the Promises you want to track.
When to use Promise.all:
Parallel Execution: You should use Promise.all when you have multiple independent asynchronous tasks that can be executed in parallel. By using
Promise.all, you can initiate all these tasks concurrently and wait for them to complete together, which can significantly improve the overall efficiency and performance of your application.
const promises = [asyncTask1(), asyncTask2(), asyncTask3()];
Promise.all(promises)
  .then((results) => {
    // All promises have resolved successfully
    // Use the results here
  })
  .catch((error) => {
    // At least one promise was rejected
    // Handle the error
  });
Dependent Operations: You can use Promise.all when you have multiple asynchronous operations that depend on the results of each other. By resolving all Promises concurrently, you can ensure that dependent operations start executing as soon as their prerequisites are met.
const fetchUserData = () => {
  return fetchUserDetails()
    .then((user) => {
      return Promise.all([fetchOrders(user.id), fetchPreferences(user.id)]);
    })
    .then(([orders, preferences]) => {
      // Use user, orders, and preferences data here
    });
};
Waiting for Multiple Resources: If your application needs to load multiple resources (like images, scripts, or data) before rendering a page,
Promise.all can be used to wait for all these resources to load before proceeding.
const loadImage = (src) => {
  return new Promise((resolve, reject) => {
    const img = new Image();
    img.src = src;
    img.onload = resolve;
    img.onerror = reject;
  });
};
const imageSources = ['image1.jpg', 'image2.jpg', 'image3.jpg'];
Promise.all(imageSources.map(loadImage))
  .then(() => {
    // All images have loaded successfully
    // Proceed with rendering
  })
  .catch((error) => {
    // Handle loading errors
  });
Error Handling: Promise.all allows you to efficiently handle errors in a batch. If any of the input Promises reject, the resulting Promise returned by
Promise.all will also reject immediately. This makes it easier to handle error scenarios when working with multiple Promises.
Remember that if any of the Promises passed to Promise.all rejects, the whole batch is considered rejected, and the
.catch block (if provided) will be executed. It's important to handle errors appropriately in your
.catch block to maintain the robustness of your application.

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What is Promise.all, and when to use it?

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