String replaceAll()

This is perhaps the feature I am most excited about. With the old approach, in order to replace all occurrences of a substring, the method replace() combined with a global regular expression has been the way to go. However, with the new method replaceAll(), we can easily return a new string that replaces all instances of a pattern without the use of complicated regex.

const myString =
  "I love Cats. Cats are supercute, especially when they are doing Catstuff";

let newString = myString.replaceAll("Cat", "Dog");

console.log(newString);

//I love Dogs. Dogs are supercute, especially when they are doing Dogstuff

This method also comes with a performance improvement as it uses string comparisons instead of regular expression matching.

Logical Assignment Operator

The Logical Assignment Operator combines Logical Operators ( &&, ||, ??) with Assignment Expressions (=).

The code example below shows this feature in use on the operators AND (&&), OR (||) and the Nullish Coalescing Operator (??) as well.

//Only assigns if left-hand side is Truthy
//Old approach
a && (a = b);

//Logical assignment operator
a &&= b;

//Only assigns if left hand-side is Falsy
//Old approach
a || (a = b);

//Logical assignment operator
a ||= b(
  //Only assigns if left hand side is Nullish
  null / undefined
);
//Old approach
a ?? (a = b);
//Logical assignment operator
a ??= b;

In the example below we have the variable money. Using the nullish coalescing operator, the defaultValue is only applied if the variable is undefined or null:

const defaultValue = 1;
let money = null;

money = money ?? defaultValue;

Using the Logical assignment operator , we get a slightly shorter syntax:

money ??= defaultValue;

Curious about Operators? Stay tuned, there might be an article about the topic further into this JavaScript Christmas calendar :angel:.

Numeric Separators

Large numbers may be challenging to read with a first glance, especially when there are repeating digits. The Numeric Separator is a useful tool that separates digits with an underscore (_) in numeric literals, thus making numeric literals more readable. The separator can be used in different positions, with as many separators as you want, in groups of any size.

const oneMillion = 1000000;
const oneMillionWithSeparators = 1_000_000;
const oneMillionAndALittleMore = 1_000_000.123_456;

As we can see, the code becomes a lot more readable. Numeric separators also work for octal integer literals

Promise.any()

In short, this method is the opposite of Promise.all(). Promise.any() takes an iterable of promise objects, and resolves if any of the supplied objects resolves. It returns a single promise, the first one to resolve, with the value from that promise.

Consider the example below, we create three promises which we feed into Promise.any().

const promise1 = new Promise((resolve) => setTimeout(resolve, 100, "first"));
const promise2 = new Promise((resolve) => setTimeout(resolve, 300, "second"));
const promise3 = new Promise((resolve) => setTimeout(resolve, 500, "third"));

const promises = [promise1, promise2, promise3];

Promise.any(promises).then((value) => console.log(value));

// Expected output: "first"

If none of the promises resolve, then a new type of exception, the AggregateError exception is thrown. AggregateError groups together individual errors into a single error in the form of an array of objects. Pretty neat!

Want to know more about promises? Visit this article from the 2019 Christmas calendar.

WeakRef

In JavaScript, references to objects are strongly held. This means that as long as a reference to the object exists, the object will not be garbage-collected but keeps the object in memory.

var a, b;
a = b = document.querySelector(".someClass");
a = undefined;
// ... GarbageCollecting...
// b is still references to the DOM-object .someClass

In scenarios where we do not want to keep objects in memory indefinitely, the WeakRef (Weak Reference) can be used to implement caches or mappings to large objects. When not in use, the memory can be garbage collected and generate a fresh cache when needed again.

A WeakRef is created using new WeakRef, and the reference is read with .deref().

const x = new WeakRef(document.querySelector(".someClass"));
const element = x.deref();

JavaScript is continuously integrating new features, and today we have looked into some of the features coming in JavaScript ES2021. For more information on proposals, and what´s coming next, take closer look here