方法1,通过正则的方式

    1. ~function(){
    2.     let myIndex = function(str) {
    3.       let reg = new RegExp(str),
    4.     res = reg.exec(this);
    5.     return res === null? -1 : res.index;
    6.   }
    8.   String.prototype.myIndex = myIndex
    9. }()

    方法2，普通的for循环

    1. ~function(
    2.     let myIndex = function(str) {
    3.         let lenT = str.length,
    4.         lenS = this.length,
    5.         res = -1;
    6.       if(lenT > lenS) {
    7.         return -1;
    8.     }
    9.     for(let i = 0;i<=lenS - lenT;i++){
    10.         if(this.slice(i,lenT+i) === str){
    11.           res = i;
    12.         break;
    13.       }
    14.     }
    15.       return res;
    16.     }
    18.     String.prototype.myIndex = myIndex
    19. ){}()