Converting a callback

2 contributors

May 20, 2020

1 minute read time

It's still pretty common to be forced to use an API that doesn't support promises. While a promise API is often preferred, it's not always available.

Before anything check:

If your library might support both callbacks and API, in the documentation, or README from Github
If there is not another library that does the same thing, but supports promises.

From a callback

Let's look at a conventional, but fictional callback API:

getWeather("Los Angeles", (error, result) => {
  if (err) throw err;

  console.log(result); // "sunny"
});

To a promise

If we want to use it as a promise, we'll need to wrap a promise around it:

function getWeatherAsync(city) {
  return new Promise((resolve, reject) => {
    getWeather(city, (error, result) => {
      if (err) reject(err);

      resolve(result);
    });
  });
}

Which we can now use:

getWeatherAsync("Los Angeles")
  .then((weather) => {
    console.log(weather);
  })
  .catch((error) => {
    console.log(error);
  });

// or using async/await
const weather = await getWeatherAsync("Los Angeles").catch((error) => {
  console.log(error);
});

console.log(weather);

In Node.js >v8.0.0 you can make use of the built-in util package which exposes a promisify method. This method helps us convert our original function to a promise-based function so that it returns the callback.

Using our example from earlier, we can write it as:

const util = require("util");

const getWeatherAsync = util.promisify(getWeather);

const weather = await getWeatherAync("Los Angeles").catch(console.log);
console.log(weather);

Note: the promisify method adds a extra argument to the arguments you passed in. In this case we are calling the custom function with a single parameter of type String, which means the original function should also accept (String, Function), Function being the callback (error, result).

Async/Await

Simplifying Promises