Promise.try()

func

A function that is called synchronously with no arguments. It can do anything—either return a value, throw an error, or return a promise.

A Promise that is:

• Already fulfilled, if func synchronously returns a value.
• Already rejected, if func synchronously throws an error.
• Asynchronously fulfilled or rejected, if func returns a promise.

You may have an API that takes a callback. The callback may be synchronous or asynchronous. You want to handle everything uniformly by wrapping the result in a promise. The most straightforward way might be Promise.resolve(func()). The problem is that if func() synchronously throws an error, this error would not be caught and turned into a rejected promise.

The common approach (lifting a function call result into a promise, fulfilled or rejected) often looks like this:

new Promise((resolve) => resolve(func()));

But Promise.try() is more helpful here:

Promise.try(func);

For the built-in Promise() constructor, errors thrown from the executor are automatically caught and turned into rejections, so these two approaches are mostly equivalent, except that Promise.try() is more concise and readable.

Note that Promise.try() is not equivalent to this, despite being highly similar:

Promise.resolve().then(func);

The difference is that the callback passed to then() is always called asynchronously, while the executor of the Promise() constructor is called synchronously. Promise.try also calls the function synchronously, and resolves the promise immediately if possible.

Promise.try(), combined with catch() and finally(), can be used to handle both synchronous and asynchronous errors in a single chain, and make promise error handling appear almost like synchronous error handling.

The following example takes a callback, "lifts" it into a promise, handles the result, and does some error handling:

function doSomething(action) {
  return Promise.try(action)
    .then((result) => console.log(result))
    .catch((error) => console.error(error))
    .finally(() => console.log("Done"));
}

doSomething(() => "Sync result");

doSomething(() => {
  throw new Error("Sync error");
});

doSomething(async () => "Async result");

doSomething(async () => {
  throw new Error("Async error");
});

In async/await, the same code would look like this:

async function doSomething(action) {
  try {
    const result = await action();
    console.log(result);
  } catch (error) {
    console.error(error);
  } finally {
    console.log("Done");
  }
}

Promise.try() is a generic method. It can be called on any constructor that implements the same signature as the Promise() constructor.

The following is a slightly more faithful approximation of the actual Promise.try() (although it should still not be used as a polyfill):

Promise.try = function (func) {
  return new this((resolve, reject) => {
    try {
      resolve(func());
    } catch (error) {
      reject(error);
    }
  });
};

Because of how Promise.try() is implemented (i.e., the try...catch), we can safely invoke Promise.try() with its this set to any custom constructor, and it will never synchronously throw an error.

class NotPromise {
  constructor(executor) {
    // The "resolve" and "reject" functions behave nothing like the native
    // promise's, but Promise.try() just calls resolve
    executor(
      (value) => console.log("Resolved", value),
      (reason) => console.log("Rejected", reason),
    );
  }
}

const p = Promise.try.call(NotPromise, () => "hello");
// Logs: Resolved hello

const p2 = Promise.try.call(NotPromise, () => {
  throw new Error("oops");
});
// Logs: Rejected Error: oops

Unlike Promise(), this NotPromise() constructor does not gracefully handle exceptions while running the executor. But despite the throw, Promise.try() still catches the exception, passing it to reject() to log out.

• Polyfill of Promise.try in core-js
• Using promises guide
• Promise
• Promise() constructor