[[3, 2], [1], [4, 12]] how to cahnage in one array javascript code example

Example 1: es6 create array with increasing number

Array.from(Array(10).keys())
//=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Example 2: list from 1 to 100 js

[...Array(100).keys()]
//=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]