Function pad

apps/json-format/format-lib.js:6–16

(source, length)

 */
Date.prototype.format = function (pattern) {
    let pad = function (source, length) {
        let pre = "",
            negative = (source < 0),
            string = String(Math.abs(source));

        if (string.length < length) {
            pre = (new Array(length - string.length + 1)).join('0');
        }

        return (negative ? "-" : "") + pre + string;
    };

    if ('string' !== typeof pattern) {
        return this.toString();