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Merging arrays

To merge arrays in JavaScript, we can either concatenate them or use the spread operator. If we nest them in an array, they stay separate inside of that wrapping array instead.

const a = ['up', 'down']
const b = ['left', 'right']
const c = ['B', 'A']

// this nests the arrays in another array instead of merging them
[a, b, c]           // ⇒ [['up', 'down'], ['left', 'right'], ['B', 'A']]

// we can concatenate many arrays at once with `.concat()`
a.concat(b, c)      // ⇒ ['up', 'down', 'left', 'right', 'B', 'A']

// the spread operator also joins the arrays into one large array
[...a, ...b, ...c]  // ⇒ ['up', 'down', 'left', 'right', 'B', 'A']