var arr = [1, 2, 3, 4]
var resulr = arr.reduce(function(v1, v2){
	return v1 + v2
},10)
console.log(result) //20

//手写一个reduce方法
var arr = [1, 2, 3, 4]

function reduce(arr, fn, initValue) {
// 	var arr2 = arr.concat([])
// 	if(initValue !== 0) {
// 		arr2.unshift(initValue)
// 	}
	//下面一行代码等效于上面三行
	var arr2 = (initValue === undefined?[]:[initValue]).concat(arr)
	while(arr2.length > 1) {
		console.log(arr2)
		arr2.splice(0, 2, fn(arr2[0],arr2[1]))
	}
		return arr2[0]
}

var result = reduce(arr, function(v1, v2){
  return v1 + v2
}, 10)

console.log(result) //20