JS-模拟printf

function printf(){
var args = [].slice.call(arguments),
fmt = args.shift(),
args_index = 0;
///%(填充的字符)?(多一共少位)?(什么格式的)/ig
return fmt.replace(/%(\w)?(\d)?([dfsx])/ig, function(_,char,num,form){
var str = num ? char.repeat(num) : '';
if(form == 'd') str += parseInt(args[args_index++]);
return num ? str.slice(num*-1) : str;
});
}
String.prototype.repeat = function(num){
// num可能是字符串，先用num-0将num变成数字，再+1
return new Array(num-0+1).join(this[0] || '');
}