Array concatenation with the spread operator

The spread operator in ES6 javascript is a swiss army knife of wonders. Previously in javascript if you wanted to concatenate two arrays you could use the concat method of an array.

let a = [1, 2, 3]
let b = [4, 5, 6]
a.concat(b) //concat though is not mutative, you have to assign the result
let c = a.concat(b) // we have concatenated

Whether or not Array.prototype.concat is mutative has always been something I get wrong. Some Array methods are mutative some are not! By using the spread operator, I never get this wrong (it's not mutative).

let c = [...[1, 2, 3], ...[4, 5, 6]]
console.log(c) // [1, 2, 3, 4, 5, 6]