What's new in ECMAScript 2019 (ES2019) / ES10

ECMAScript (ES for short) is a scripting language specification standardized by ECMA International in ECMA-262 and ISO/IEC 16262. It was created to standardize the JavaScript language, so as to foster multiple independent standard implementations from browser vendors. It evolves every year with new features.

The 2019 edition of the ECMAScript specification saw the addition of many new features, and here I’ll cover some of these new features. I personally love how javaScript keeps evolving and improving on a regular basis.

Array.flat()

Array.flat() returns a new array with any sub-array(s) flattened. A call to Array.flat() without any arguments will only flatten one-level deep. An optional depth argument can be provided or it can just be called consecutively.

Examples:

let arr = [1, 2, 3, [4, 5, 6, [7, 8, 9, [10, 11, 12]]]];

arr.flat(); // [1, 2, 3, 4, 5, 6, Array(4)];

arr.flat().flat(); // [1, 2, 3, 4, 5, 6, 7, 8, 9, Array(3)];

arr.flat(3); // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]

// Or, if you're not sure about the depth of the array:
arr.flat(Infinity); // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]

Array.flatMap()

The flatMap() method is identical to the ES6 map method, but also flattens at the same time. The flatMap() method first maps each element using a mapping function, then flattens the result into a new array. flatMap() is often quite useful, as merging both into one method is slightly more efficient.

Examples:

let arr = [1, 2, 3, 4, 5];

arr.map(x => [x, x * 2]);
// [Array(2), Array(2), Array(2)]
// 0: (2)[1, 2]
// 1: (2)[2, 4]
// 2: (2)[3, 6]

arr.flatMap(v => [v, v * 2]);
// [1, 2, 2, 4, 3, 6, 4, 8, 5, 10]

String.trimStart() & String.trimEnd()

String.trimStart() can be used to trim white space from the start of a string.

Examples:

let  greeting =  "    Hello everyone";

console.log(greeting.trimStart());
// "Hello everyone"

let greeting = "Hello world    ";

console.log(greeting.trimEnd());
// "Hello world"

Optional Catch Binding

Optional catch binding allows developers to use try/catch without the error parameter inside the catch block.

Examples:

Before ES2019 we use:

try {
  // some code
}
catch (err) {
  // error handling code
}

Now we can use try/catch like this with ES2019:

try  {
  // some code
}
catch {
  // error handling code
}

Object.fromEntries()

It creates an object or transforms key-value pairs into an object. It only accepts iterables e.g: Object.fromEntries(someIterable).

Examples:

let entries = new Map([["name", "john"], ["age", 22]]);

console.log(Object.fromEntries(entries));
// { name: 'john', age: 22 }

Symbol.description

The read-only description property is a string returning the optional description of Symbol objects.

Examples:

let mySymbol = `My Symbol`;

let symObj = Symbol(mySymbol);

console.log(symObj) // Symbol(mySymbol);

console.log(String(symObj) === `Symbol(${mySymbol})`); // true

console.log(symObj.description); // "My Symbol"