mask.js

export const mask = function (m, str) {
    if (str === undefined || str === null) return '';
    var m,
        l = (m = m.split("")).length,
        s = str.split(""),
        j = 0,
        h = "";

    for (var i = -1; ++i < l;)
        if (m[i] != "#") {
            if (m[i] == "\\" && (h += m[++i])) continue;
            h += m[i];
            i + 1 == l && (s[j - 1] += h, h = "");
        }
        else {
            if (!s[j] && !(h = "")) break;
            (s[j] = h + s[j++]) && (h = "");
        }
    return s.join("") + h;
};