JS字符串补全方法padStart()和padEnd()

var t = new Date().getMonth() + 1;
// t ===> 7,没有0，怎么展示成下面的样子？
// 2018-07-23

// 补0函数
function(num) {
    return +num < 10 ? '0' + num : num;
}

var a = '7';
a.padStart(2, '0');
// a ===> 07

var b = 'hi';
b.padStart(10, 'hello');
// b ===> hellohelhi, 重复补充

var c = 'hi';
c.padStart(10, '');
// c ===> hi

var d = 'a';
d.padStart(5, false);
// falsa
d.padStart(5, []);
// a
d.padStart(5, {});
// [obja
d.padStart(5, null);
// nulla
d.padStart(5, function(){});
// funca

var a = '22'
a.padStart(len, str)

var res = ''
if (str.toString) {
    res = str.toString();
}
else {
    res = Object.prototype.toString.call(str);
}
return res;

res.repeat(len/2+1).slice(0, len - a.length) + a

var a = 'aaa'.padEnd(15, {});
// aaa[object Obje

if (!String.prototype.padStart) {
    String.prototype.padStart = function padStart(targetLength,padString) {
        targetLength = targetLength>>0; //truncate if number or convert non-number to 0;
        padString = String((typeof padString !== 'undefined' ? padString : ' '));
        if (this.length > targetLength) {
            return String(this);
        }
        else {
            targetLength = targetLength-this.length;
            if (targetLength > padString.length) {
                padString += padString.repeat(targetLength/padString.length); //append to original to ensure we are longer than needed
            }
            return padString.slice(0,targetLength) + String(this);
        }
    };
}