mergeSort.js

function mergeSort(array) {
    for (var b = 1; b < array.length; b *= 2) {

        let a = []
        for (var i = 0; i < array.length; i += 2 * b) {
            
            a = a.concat(
                merger(
                    array.slice(i, i + b),
                    array.slice(i + b, i + 2 * b)
                )
            )
        }

        array = a
    }

    return array
}

function merger(a1, a2) {
    let l = 0
    let r = 0

    let merged = []
    while (l < a1.length && r < a2.length) {
        if (a1[l] <= a2[r]) {
            merged.push(a1[l++])
        } else {
            merged.push(a2[r++])
        }
    }

    while (l < a1.length) {
        merged.push(a1[l++])
    }

    while (r < a2.length) {
        merged.push(a2[r++])
    }

    return merged
}