cancelByPair.js

// Reduce a list by cancelling pairs of values.
// e.g. cancelByPair('LLLRLRLRLR', 'L', 'R') === 'LL'
function cancelByPair(list, firstValue, secondValue) {

    // First filter out all of these values, and count them as we go.
    let count = 0;
    const cancelledList = list.filter(c => {
        switch (c) {
            case firstValue:
                count++
                return false

            case secondValue:
                count--
                return false

            default:
                return true
        }
    })
    // Now, use the count to rebuild the list with the surplus value.
    .concat(
        // Construct an array of the size of count, so we may iterate.
        Array.from(Array(Math.abs(count)).keys())
        // If count is greater than zero, we know we removed too many of the first value.
        .map(_ => count >= 0 ? firstValue : secondValue))

    return cancelledList
}