Best of Modern JavaScript — Composing Promises

Best of Modern JavaScript — Composing Promises

Different ways to run multiple promises.

Since 2015, JavaScript has improved immensely.

It’s much more pleasant to use it now than ever.

In this article, we’ll look at handling JavaScript promise exceptions.

Handling Exceptions in Promise-Based Functions

We can handle exceptions in promise-based functions.

If exceptions are thrown in then and catch callbacks, then the methods convert them into rejections.

However, if we run the synchronous code before the promise code, then the whole function throws an exception.

This may be a problem that we want to solve.

One way to solve this is to surround the synchronous and promise code with try-catch.

For example, we can write:

function foo() {
  try {
    syncFn();
    return asyncFn()
      .then(result => {
        //...
      });
  } catch (err) {
    return Promise.reject(err);
  }
}

We have the foo function with the try-catch block which runs syncFn first and then asyncFn which returns a promise.

We return the promise chain in the try block.

And if the catch block is run, then we return a rejected promise.

This lets us keep the function async all the way through.

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