循环原理

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1. forEach

      let arr = [1,2,3,4];
      Array.prototype.myForEach = function(fn) {
        for(let i = 0 ; i < this.length ; i++) {
          fn(this[i] , i , this);
        }
      }
      arr.myForEach(function(currentVal , currentIndex , arr) {
        console.log(currentVal , currentIndex , arr);
      })