reduce的高级用法

计算数组中每个元素出现的次数

const arr = ['tom', 'alice', 'zoey', 'rain', 'tom']
const num = arr.reduce((prev, cur) => {
  if(cur in prev) {
    prev[cur] ++
  } else {
    prev[cur] = 1
  }

  return prev
}, {})
console.log(num); //{tom: 2, alice: 1, zoey: 1, rain: 1}

数组去重

const arr = ['tom', 'alice', 'zoey', 'rain', 'tom']
const newArr = arr.reduce((prev, curr) => {
  if(!prev.includes(curr)) {
    prev.push(curr)
  }
  return prev
}, [])
console.log(newArr); // ["tom", "alice", "zoey", "rain"]

数组扁平化

const arr = [['tom', 'alice'], ['zoey', ['rain', 'tom']]]
const oneDimensionalArr = (arr) => {
  return arr.reduce((prev, curr) => {
    return prev.concat(Array.isArray(curr) ? oneDimensionalArr(curr) : curr)
  }, [])
}
console.log(oneDimensionalArr(arr)); // ["tom", "alice", "zoey", "rain", "tom"]