Shift an array to left or right

shiftArray.js

// +by shift left
// -by shift right
const shiftArray = (arr, by) => {
    const _by = (by | 0) % arr.length;
    const rest = by > 0 ? arr.slice(0, _by) : arr.slice(_by);
    return arr.map(
        (_, i) => arr[i + _by] !== undefined 
            ? arr[i + _by] 
            : rest.shift()
	);
};

// Example:
// const arr = [1, 2, 3, 4]
// shiftArray(arr, 1)  -> [2, 3, 4, 1]
// shiftArray(arr, 2)  -> [3, 4, 1, 2]
// shiftArray(arr, -1) -> [4, 1, 2, 3]
// shiftArray(arr, -2) -> [3, 4, 1, 2]