Today I learned about Array.prototype.fill and friends.

You can use the method to fill an array with values, or create a new array filled with values.

Pitfall: if you fill a new array with a value that is an object, each slot in the array will reference that object.

const newArr = Array(4).fill([]) > [[], [], [], []]
newArr[1].push(8) > 1
newArr > [[8], [8], [8], [8]]

Further Reading