数组及对象实用方法总结

在项目中经常需要处理数据，经常需要操作数组和对象， 且正确使用reduce能够节省很多不必要的代码

1. 造假数据，例如数组

1.1 使用repeat方法：

'abc'.repeat(3).split('')
// ["a", "b", "c", "a", "b", "c", "a", "b", "c"]

1.2 使用fill方法：

Array(5).fill('abc')
// ["abc", "abc", "abc", "abc", "abc"]

2. 统计数组中相同项出现的次数：

const arr = ['mo', 'melantha', 'mo', 'nian', 'chen', 'melantha'];

const nameArr = arr.reduce((prev, name) => {
    prev[name]  = prev[name] ? ++prev[name] : 1;
    return prev
}, {});
//  {mo: 2, melantha: 2, nian: 1, chen: 1}

3. 把数组中按相同元素归类： eg： 把dateArr数组中的日期，出现在同一天的进行归类

const dateArr = [
    {
        date: '2019-06-12',
        count: 5
    },
    {
        date: '2019-06-13',
        count: 4
    },
    {
        date: '2019-05-12',
        count: 3
    },
    {
        date: '2019-06-12',
        count: 8
    },
    {
        date: '2019-06-13',
        count: 9
    },
    {
        date: '2019-04-12',
        count: 1
    },
];

const  dateObj = dateArr.reduce((prev, item) => {
    if (prev[item.date]) {
        prev[item.date].push(item)
    } else {
        prev[item.date] = [item]
    }
    return prev
}, {});

image.png

4. 将多维数组平铺到一维数组

const flatten = (arr) => {
   const result = arr.reduce((prev, item) => {
      if(Array.isArray(item)) {
        return prev.concat(flatten(item))
    } else {
        return prev.concat(item)
    }
   }, [])
  return result;
}
flatten(['a',  [1,2,3, ['c', [9, [3], 10],'k']], ['4', '7']]);
// ["a", 1, 2, 3, "c", 9, 3, 10, "k", "4", "7"]

5. 将多维数组平铺到指定深度

const flatten = (arr, depth = 1) => {
    if ( depth !== 1) {
        const value1 = arr.reduce((prev, item) => {
        if(Array.isArray(item)) {
          return prev.concat(flatten(item, depth - 1));
       } else {
          return prev.concat(item);
      }
    } ,[]);
    return value1;
    } else {
    const value2 = arr.reduce((prev, item) => {
        return prev.concat(item)
    }, []);
    return value2;
  }
}
flatten(['a',  [1,2,3, ['c', [9, [3], 10],'k']], ['4', '7']], 4)
// ["a", 1, 2, 3, "c", 9, 3, 10, "k", "4", "7"]
flatten(['a',  [1,2,3, ['c', [9, [3], 10],'k']], ['4', '7']], 2)
//  ["a", 1, 2, 3, "c", [9, [3], 10] , "k", "4", "7"]