reducer 函数接收4个参数:
Accumulator (acc) (累计器)
Current Value (cur) (当前值)
Current Index (idx) (当前索引)
Source Array (src) (源数组)
数组所有数值的和
const sum = [1,2,3,4].reduce((accum, current) => {
console.log("accum, current", accum, current)
return accum + current
}, 0)
console.log(sum)
// accum累计器是每次reduce计算后的值， 这里给了初始值0
image.png
上面的代码把初始值改成1：
const sum = [1,2,3,4].reduce((accum, current) => {
console.log("accum, current", accum, current)
return accum + current
}, 1)
console.log(sum)
image.png
相应最后的和也加了1
然后接下来把初始值去掉
const sum = [1,2,3,4].reduce((accum, current) => {
console.log("accum, current", accum, current)
return accum + current
})
console.log(sum)
image.png
只运行了3次，就得到了最后的正确结果
MSD里面有说明这种情况
注意：如果没有提供initialValue，reduce 会从索引1的地方开始执行 callback 方法，跳过第一个索引。如果提供initialValue，从>索引0开始。
数组去重
let chongfuArr = [
{
id: 1,
name: '测试1'
},
{
id: 1,
name: '测试1'
},
{
id: 2,
name: '测试2'
},
{
id: 3,
name: '测试3'
},
{
id: 3,
name: '测试3'
},
{
id: 4,
name: '测试4'
}
]
let obj = {}
const test = chongfuArr.reduce((item, next) => {
// console.log('item, next', item, next)
if(!obj[next.id]){
item.push(next)
obj[next.id] = true
}
return item
}, [])
console.log("去重后的数组", test)
// [
{ id: 1, name: '测试1' },
{ id: 2, name: '测试2' },
{ id: 3, name: '测试3' },
{ id: 4, name: '测试4' }
]
