JS 中的substring ,substr ,slice,split，join

1. substr with different arguments passed in：
   1. str.substring(startNum,stopNum );
   2. str.slice(startNum,stopNum );
   3. str.substr(startNum,length );
2. substring vs slice
   1. start = stop ,return ‘ ‘; the same.
   2. start or stop bigger than str.length, it will make the bigger one equal str.length.
   3. start < 0 || stop < 0,  substring will make it equal 0, but slice won’t do like this.
   4. start > stop ,substring will swap those two arguments, but slice won’t do like this.
3. str.split(‘&'，num )  < ==  == > array.join(‘&’)；