function myReduce (fn,baseNumber) {
// 判断是不是个数组
if (Object.prototype.toString.call(this) !== '[object Array]') {
throw new TypeError('not a array')
}
// 判断传入的是不是个函数
if(typeof fn !== 'function') throw new Error(`${fn} is not a function`)
// 判断数组是否为空
if(!this.length) throw new Error(`Reduce of empty array with no initial value`)
// 判断传回的基数，是否有值，没有值的话拿数组的第一个值为基数
baseNumber = (baseNumber || 0) + this[0]
let index = 0
//循环执行遍历执行fn函数
while(index < this.length) {
console.log(index)
baseNumber = fn(baseNumber,this[index+1] || 0, index, this)
index++
}
return baseNumber
}
Array.prototype.myReduce = myReduce
a = [1,2,3,4,5,6]
const x=
a.myReduce(function(a,b){
return a+b
},20)
console.log(x)
