reduce 函数的功能，如何实现的，动手实现一下

[lgwebdream]

[lgwebdream]

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[gaohan1994]

  const reduce = (source, callback, initValue) => {
    let acc = initValue;

    for (let index = 0; index < source.length; index++) {
      acc = callback(acc, source[index], index, source);
    }

    return acc;
  };

[Alicca-miao]

 Array.prototype.myreduce=function(fn,initial) {
        let st=0
       if(typeof initial==='undefined'){
        st=1
        initial=this[0]
       }
        for(let i=st;i<this.length;i++) {
            
     initial=fn(initial,this[i],i,this)
        }
        return initial

       }

       let arr=[1,2,3,4]
       let z=arr.myreduce((pre,cur)=>pre+cur)
       console.log(z)

[ZhangDaZongWei]

Array.prototype.myReducer = function (fn, initialValue) {
  const arr = this;

  let prev = initialValue;

  arr.forEach(item => {
    prev = fn(prev, item);
  });

  return prev;
}