``` 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] || ‘‘); } ```