Copy an existing array by value
const b = Array.from(a)
const b = Array.of(...a)
Copy just some values from an existing array
const b = Array.from(a, x => x % 2 == 0)
Copy portions of an array into the array itself, in other positions
const a = [1, 2, 3, 4]
a.copyWithin(0, 2) // [3, 4, 3, 4]
const b = [1, 2, 3, 4, 5]
b.copyWithin(0, 2) // [3, 4, 5, 4, 5]
//0 is where to start copying into,
// 2 is where to start copying from
const c = [1, 2, 3, 4, 5]
c.copyWithin(0, 2, 4) // [3, 4, 3, 4, 5]
//4 is an end index
The spread operator
You can expand an array, an object or a string using the spread operator ....
Let’s start with an array example. Given
const a = [1, 2, 3]
you can create a new array using
const b = [...a, 4, 5, 6]
You can also create a copy of an array using
const c = [...a]
This operator has some pretty useful applications. The most important one is the ability to use an array as function argument in a very simple way:
const f = (foo, bar) => {}
const a = [1, 2]
f(...a)
(in the past you could do this using f.apply(null, a) but that’s not as nice and readable)