Description

Array.prototype.flat uses 1 as the argument if you pass undefined or do not provide a value.
This means that .flat(1) is the same as .flat(), making the argument unnecessary.

Fail

array.flat(1);

const one = 1;
array.flat(one);

Pass

array.flat();

const one = 1;
array.flat();

let levels = 1; // Mutable variable, might be reassigned later
array.flat(levels);