Spread & Rest Syntax

Spread expands an iterable into individual elements.

const a = [1,2];
const b = [...a,3,4]; // [1,2,3,4]

Rest gathers remaining elements into an array.

function sum(...nums){ return nums.reduce((a,b)=>a+b,0); }

Works with objects as well.

const obj1 = {a:1};
const obj2 = {...obj1, b:2}; // {a:1,b:2}