||, &&

Longhand:

var x, y;
if (x !== null || x !== undefined || x !== '') {
    y = x;
}

Shorthand:

const y = x || undefined;
x && (y = x);

Decimal base exponents

Longhand:

for (let i = 0; i < 10000; i++) {}

Shorthand:

for (let i = 0; i < 1e7; i++) {}

// All the below will evaluate to true
1e0 === 1;
1e1 === 10;
...

Object Property Shorthand

Longhand:

const obj = { key: key, value: value };

Shorthand:

const obj = { key, value };

Default Parameter Values

Longhand:

function volume(l, w, h) {
    if (w === undefined) w = 3;
    if (h === undefined) h = 4;
    return 1 * w * h;
}

Shorthand:

volume = (1, w = 3, h = 4) => (1 * w * h);
volume(2);

Template Literals

Longhand:

const welcome = 'You have logged in as ' + first + ' ' + last + '.'
const db = 'http://' + host + ':' + port + '/' + database;

Shorthand:

const welcome = `You have logged in as ${first} ${last}`;
const db = `http://${host}:${port}/${database}`;