Unlocking the Power of Promise.allSettled() in TypeScript

In the world of asynchronous JavaScript and TypeScript, promises are indispensable for managing asynchronous operations. You’re likely familiar with the Promise.all() method, which is great for handling multiple promises concurrently.

But what if you need a more resilient approach that doesn’t abort when one of the promises fails? Enter Promise.allSettled().

In this article, we’ll explore Promise.allSettled() and how to use it effectively in TypeScript. We’ll also address a common issue developers encounter when trying to use this method in TypeScript with VS Code.

Understanding Promise.allSettled()

Promise.allSettled() is a method that generates a single promise from an array of promises, similar to Promise.all(). However, it has a crucial difference: it runs all the promises passed to it, regardless of whether any of them get rejected.

In a typical scenario using Promise.all(), if one promise in the array is rejected, the entire operation is aborted, and you’ll receive a rejection.

This behavior can be problematic in situations where you want to handle each promise’s outcome individually without halting the entire process.

Promise.allSettled() solves this problem by executing all the promises and returning an array of objects representing the state of each promise.

These objects contain two properties: status (either “fulfilled” or “rejected”) and value (the promise’s result or reason for rejection).

Using Promise.allSettled() in TypeScript

Let’s say you’re building a Node.js Express application using TypeScript, and you encounter a scenario where you want to fetch data from multiple sources concurrently.

Promise.all() seems like a suitable choice, but what if one of the sources is temporarily unavailable, causing a rejection? This is where Promise.allSettled() comes to the rescue.

Here’s an example of how you can use Promise.allSettled():

// Define functions to fetch data from different sources
async function fetchDataFromSourceA() {
  // Simulate fetching data from Source A
  return "Data from Source A";
}
async function fetchDataFromSourceB() {
  // Simulate fetching data from Source B
  return new Promise((resolve, reject) => {
    // Simulating temporary unavailability
    setTimeout(() => {
      reject("Error: Source B is temporarily unavailable");
    }, 2000); // Simulate a delay of 2 seconds
  });
}
async function fetchDataFromSourceC() {
  // Simulate fetching data from Source C
  return "Data from Source C";
}
// Main function to fetch data from all sources concurrently
async function fetchDataFromMultipleSources() {
  const promises = [
    fetchDataFromSourceA(),
    fetchDataFromSourceB(),
    fetchDataFromSourceC(),
  ];
  const results = await Promise.allSettled(promises);
  results.forEach((result, index) => {
    if (result.status === 'fulfilled') {
      console.log("Data from source " + (index + 1) + ":", result.value);
    } else {
      console.error("Error fetching data from source " + (index + 1) + ":", result.reason);
    }
  });
}
// Call the main function
fetchDataFromMultipleSources();

This code snippet fetches data from multiple sources using promises. Even if one source fails, the code continues to execute and reports the outcome of each promise individually.

Discover an insightful article: Exploring TypeScript Decorators: A Comprehensive Guide, unraveling the power of decorators in TypeScript for enhanced code organization and functionality get back to our discussion.

Resolving TypeScript Errors

Now, let’s address a common issue developers encounter when trying to use Promise.allSettled() in TypeScript with the Visual Studio Code (VS Code) IDE.

When you attempt to use Promise.allSettled(), you might encounter the following error:

Property 'allSettled' does not exist on type 'PromiseConstructor'

To resolve this error and make Promise.allSettled() available in your TypeScript project, you need to specify the appropriate TypeScript library.

Here are the steps to resolve the error:

• Open your tsconfig.json file.
• Under the compilerOptions section, locate the lib property.
• Add “es2020.promise” to the list of libraries, like this:

{
  "compilerOptions": {
    "target": "ES2020",  
    "lib": ["ES2020", "DOM"],  
    "module": "commonjs", 
  }
}

By including es2020.promise you’re telling TypeScript to recognize and support Promise.allSettled().

Alternatively, you can use the es2020 library, which includes all features from ECMAScript 2020, including Promise.allSettled():

{

  "compilerOptions": {

    "lib": [

      "es2020"

    ]

    // ...

  }

}

Finally, if you use VS Code as your IDE and TypeScript still doesn’t recognize Promise.allSettled(), try closing and reopening VS Code.

This often resolves IntelliSense issues, and you should no longer see the error. Additionally, you’ll have the allSettled() option available via IntelliSense as you type Promise.

People also read about discovering the contrasts between Types vs Interfaces in TypeScript, TypeScript Optional Parameters, and TS Omit guiding us through their impact on code scalability. Let’s return to our main discussion afterward.

Conclusion

Promise.allSettled() is a powerful addition to the JavaScript and TypeScript toolset. It empowers developers to work with multiple promises concurrently while gracefully handling both successful and failed promises.

By understanding how to use Promise.allSettled() and resolving TypeScript configuration issues, you can leverage this method to build more robust and resilient asynchronous code in your TypeScript projects.