A Recap of Upcoming Features in ECMAScript 2022 (ES13)

As promised since 2016, ECMAScript has committed to releasing a new version of JavaScript every year, with improvements and new features. It's now 2022, so what are the upcoming features? Let's recap some of the highlights.

Private slots

JavaScript has long supported Class syntax, but there are still some shortcomings. One of the features in ES13 is the introduction of private slots in Classes.

class Person {
  #name;

  getName() {
      return this.#name;
  }

  setName(name) {
      this.#name = name;
  }
}

With private slots, you can no longer access the name property directly; instead, you must use get/set methods.

const p = new Person();
p.setName('2coffee.dev');
p.name; // undefined
p.getName(); // 2coffee.dev

Additionally, you can use the Object.hasOwn function to determine if a Class contains a specific private property.

Object.hasOwn(Person, 'name'); // true

Top-level await

Previously, if you wanted to use await, you had to use it within an async function. Now, you can use await at the top level, outside of an async function.

async function fetchData() {
  const resp = await fetch('https://jsonplaceholder.typicode.com/users');
  return resp.json();
}
fetchData();

# Now, you can do
const resp = await fetch('https://jsonplaceholder.typicode.com/users');
resp.json();

This feature is also useful with the import syntax. As we know, import is executed asynchronously to make module loading faster. However, it requires writing additional code to wait for the module to be loaded and ready to use its functions. With ES13, the import syntax will work "as if" it's synchronous by using Promise.all for all imports.

Let's assume two modules, first.mjs and second.mjs, export asynchronous functions:

# first.mjs
async function first() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve('first');
    }, 1000);
  });
}

export default first();

Then, use these modules:

import { first } from './first.mjs';
import { second } from './second.mjs';
console.log(first, second);

# will be equivalent to
import { promise as firstPromise, first } from './first.mjs';
import { promise as secondPromise, second } from './second.mjs';
export const promise = (async () => {
  await Promise.all([firstPromise, secondPromise]);
  console.log(first, second);
})();

Error Cause

Error cause is a new approach to error handling in JavaScript. Consider the following code snippet for fetching data:

const fetchData = async () => {
  return await fetch(url)
    .catch(err => {
        // What will you do with err?  
    });
}

What will you do with err? Will you log it and throw a new error message saying "Failed to fetch data"?

const fetchData = async () => {
  return await fetch(url)
    .catch(err => {
        console.log(err);
        throw new Error('Failed to fetch data');
    });
}

When calling fetchData and encountering an error, you will receive new Error('Failed to fetch data') instead of the original err. This makes it challenging to trace the error when using nested catch blocks, as the resultant error message may not be the expected err, but a different error message.

With ES13, you can use Error Cause to throw new Error('Failed to fetch data') while also including the err itself:

const fetchData = async () => {
  return await fetch(url)
    .catch(err => {
        throw new Error('Failed to fetch data', { cause: err });
    });
}

You can then access err in fetchData as follows:

try {
  resp = await fetchData();
  // ...  
} catch(e) {
  console.log('cause', e.cause);
}

.at() Method

Use the .at() method to retrieve the element at the specified index. You can also use a negative index to retrieve positions from right to left.

['a', 'b', 'c'].at(0); // a
['a', 'b', 'c'].at(-1); // c

This method applies to both arrays and strings.

RegExp match indices

Add the /d flag to a regex to capture the start and end indices of the matching string. These indices are stored in the indices property.

const matchObj = /(a+)(b+)/d.exec('aaaabb');
console.log(matchObj.indices); // [[0, 6], [0, 4], [4, 6]]

Conclusion

These are some of the highlights of the upcoming features in ECMAScript 2022. ES13 is expected to be released around June 2022. Stay updated!

References:

• https://exploringjs.com/impatient-js/ch_new-javascript-features.html#new-in-es2022
• https://blog.saeloun.com/2021/11/25/ecmascript-top-level-await
• https://dev.to/hemanth/error-cause-in-javascript-425d