ECMAScript (ES) is the specification on which JavaScript is based. This language is constantly evolving and receiving updates every year. It is useful to keep up to avoid being stumped by syntax and stay productive in any JavaScript codebase.

ES2019 is available in the latest version of Chrome, Firefox, Safari, and Node. It is also available in older platforms via compilation with Babel or Typescript. Remember to always check Can I Use website to make sure the new feature you want to use is available in your target platforms.

Array.flat()
Array.flatMap()
String.trimStart()
String.trimEnd()
Object.fromEntries
Optional Catch Binding
Symbol.prototype.description
Function.prototype.toString()
JSON improvements

Array.flat()

The flat() method returns a new array flattering all the sub-array elements recursively up to the specified depth.

const a1 = [1, 2, [3, 4], 5];
a1.flat(); // [1, 2, 3, 4, 5]

By default it only flats one level flat(1), but you can convert any Array multi-dimensional to one-dimension using Infinity.

const a2 = [1, 2, [[3, [4, 5]], 6], 7];
a2.flat(Infinity); // [1, 2, 3, 4, 5, 6, 7]

And, if you have an empty element as part of you Array, it'll be removed in the new Array

const a3 = [1, 2, , 5];
a3.flat(); // [1, 2, 5]

Array.flatMap()

It returns a new array combining flat() and map() methods. Remember: ES6 map() method of an Array returns a new Array applying a function to each of its elements.

Suppose we want to double each element of an array and put the result after each one of these elements:

const b1 = [2, 8, 32];
const b2 = b1.map(el => {
  return [el, el * 2];
});
b2; // [[2, 4], [8, 16], [32, 64]]
const b3 = b2.flat();
b3; // [2, 4, 8, 16, 32, 64]

Using flatMap()

const b4 = [2, 8, 32];
const b5 = b4.flatMap(el => {
  return [el, el * 2];
});
b5; // [2, 4, 8, 16, 32, 64]

String.trimStart() and String.trimEnd()

String.trim() removes the whitespaces from the beginning and the end of a String so using String.trimStart() will only remove the whitespaces from the beginning of the string while use String.trimEnd() will do but with the whitespaces from the end.

const c1 = '      Start here.      ';
const c2 = c1.trimStart();
c2; // 'Start here.      ' (with the spaces at the end)

const c3 = '      End here.      ';
const c4 = c3.trimEnd();
c4; // '      End here.' (with the spaces at the beggining)

Object.fromEntries(iterable)

This method only accepts an iterable as argument and it creates an Object from a list of key-value pairs e.g. produced by Object.entries() (ES2017) which is used to transform an Object to an Array of [key, value] pairs.

Suppose we want double each value of the properties of an object:

const d1 = {
  prop1: 2,
  prop2: 8,
  prop3: 32
};
const d2 = Object.entries(d1);
d2; // [["prop1", 2], ["prop2", 8], ["prop3", 32]]

With map() we can iterate over each element and apply the function to double each value:

const d3 = d2.map(([key, value]) => [key, value * 2]);
d3; // [["prop1", 4], ["prop2", 16], ["prop3", 64]]

Now, using fromEntries() method we will generate the Object with the values changed.

const d4 = Object.fromEntries(d3);
d4;
/*
{
  prop1: 4,
  prop2: 16,
  prop3: 64,
}
*/

Optional Catch Binding

It allows to use the catch() clause without the need to specify the parameter (the exception).

try {
  // execute code 
} catch {
  // handle error
}

There is no need to add the parameter to the catch() (optional)

try {
  // execute code
} catch (err) {
  // handle error
}

Symbol.prototype.description

It allows to provide a description when creating a Symbol via the factory function Symbol() and access to it via the description property.

const e1 = Symbol('Optional description');
e1.description; // 'Optional description'

Function.prototype.toString()

It returns the function exactly as it was defined including withespaces and comments.

Previous ES10

const fn = function /* initial test */ test() {};
fn.toString(); // 'function test() {}'

Now it will be:

const fn = function /* initial test */ test() {};
fn.toString(); // 'function /* initial test */ test() {}'

JSON improvements

The line separator \u2028 and paragraph separator \u2029 symbols now parse correctly instead of resulting in a SyntaxError when using using JSON.parse()

Well-formed JSON.stringify

Prevent JSON.stringify from returning ill-formed strings (specifically, surrogate code points U+D800 through U+DFFF).

# ES10

What's new in ES2019