ES10 - ECMAScript 2019

Features of ES10

Array.Flat()

let multi = [1,2,3,[4,5,6,[7,8,9,[10,11,12]]]];
multi.flat();               // [1,2,3,4,5,6,Array(4)]
multi.flat().flat();        // [1,2,3,4,5,6,7,8,9,Array(3)]
multi.flat().flat().flat(); // [1,2,3,4,5,6,7,8,9,10,11,12]
multi.flat(Infinity);       // [1,2,3,4,5,6,7,8,9,10,11,12]

Array.flatMap()

let array = [1, 2, 3, 4, 5];
array.map(x => [x, x * 2]);

[Array(2), Array(2), Array(2)]
0: (2)[1, 2]
1: (2)[2, 4]
2: (2)[3, 6]
3: (2)[4, 8]
4: (2)[5, 10]

Object.fromEntries()

let obj = { apple : 10, orange : 20, banana : 30 };
let entries = Object.entries(obj);
entries:
(3) [Array(2), Array(2), Array(2)]
 0: (2) ["apple", 10]
 1: (2) ["orange", 20]
 2: (2) ["banana", 30]
let fromEntries = Object.fromEntries(entries);
{ apple: 10, orange: 20, banana: 30 }

String.trimStart() & String.trimEnd()

let greeting = "     Space around     ";
greeting.trimEnd();   // "     Space around";
greeting.trimStart(); // "Space around     ";

Optional Catch Binding

Allow developers to use try/catch without creating an unused binding. You are free to go ahead make use of catch block without a param

Previous:
(() => {
    try {
        JSON.parse(text)
        return true
    } catch(err) {
        return false
    }
})()
=> false

New feature:
try {
    JSON.parse(text);
    return true;
}
catch
{
    return false;
}

Function.toString()

The toString() method returns a string representing the source code of the function.Earlier white spaces,new lines and comments will be removed when you do now they are retained with original source code

function () { console.log('Hello there.'); }.toString();
// function () { console.log('Hello there.'); }

Symbol.description

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

Symbol('desc').toString();   // "Symbol(desc)"
Symbol('desc').description;  // "desc"
Symbol('').description;      // ""
Symbol().description;        // undefined

// well-known symbols
Symbol.iterator.toString();  // "Symbol(Symbol.iterator)"
Symbol.iterator.description; // "Symbol.iterator"

// global symbols
Symbol.for('foo').toString();  // "Symbol(foo)"
Symbol.for('foo').description; // "foo"

Well Formed JSON.Stringify()

To prevent JSON.stringify from returning ill-formed Unicode strings.

Before:
    JSON.stringify("\uD800"); // '"�"'
    
Now:
    JSON.stringify("\uD800"); // '"\\ud800"'

Array.Sort Stability

Users with same rating retain their sorting order

var fruit = [
    { name: "Apple",      count: 13, },
    { name: "Pear",       count: 12, },
    { name: "Banana",     count: 12, },
    { name: "Strawberry", count: 11, },
    { name: "Cherry",     count: 11, },
    { name: "Blackberry", count: 10, },
    { name: "Pineapple",  count: 10, }
];
// Create our own sort criteria function:
let my_sort = (a, b) => a.count - b.count;
// Perform stable ES10 sort:
let sorted = fruit.sort(my_sort);
console.log(sorted);