var str = 'What a good day today!';

 1 //方法一

 2 function returnString1(str){

 3     var arr = str.split(' ');

 4     var num = [];

 5     // console.log(arr);

 6     for(var i=0,len=arr.length; i<len; i++){

 7         // console.log(arr[i].length);

 8         num.push(arr[i].length);

 9     }

10     num.sort(sortNum);

11     return num.shift();

12 }

13 function sortNum(a, b){

14     if(a > b){

15         return 1;

16     }else if(a < b){

17         return -1;

18     }else {

19         return 0;

20     }

21 }

22 console.log(returnString1(str));

 1 //方法二

 2 function returnString2(str){

 3     var num = 0;

 4     var arr = str.split(" ");

 5     arr.forEach(function(item,i){

 6         if(i === 0){

 7             num = item.length;

 8         }else{

 9             if(item.length<num){

10                 num = item.length;

11             }

12         }

13     });

14     return num;

15 }

16 console.log(returnString2(str));

1 //方法三

2 function returnString3(str){

3     return Math.min.apply(null, str.split(' ').map(w => w.length));

4 }

5 console.log(returnString3(str));