将平行结构转换为文件树结构

var arr = [
  {
    id: 0,
    parentId: null,
    children: [],
  },
  {
    id: 1,
    parentId: 0,
    children: [],
  },
  {
    id: 2,
    parentId: 0,
    children: [],
  },
  {
    id: 3,
    parentId: 1,
    children: [],
  },
  {
    id: 4,
    parentId: 1,
    children: [],
  },
  {
    id: 5,
    parentId: 1,
    children: [],
  },
  {
    id: 6,
    parentId: 1,
    children: [],
  },
  {
    id: 7,
    parentId: 2,
    children: [],
  },
  {
    id: 8,
    parentId: 2,
    children: [],
  },
  {
    id: 9,
    parentId: 2,
    children: [],
  },
  {
    id: 10,
    parentId: 2,
    children: [],
  },
];
function multiArray(arr) {        //一维转多维
  const newArr = [];
  arr.forEach(function (item, index) {
    if (item.parentId === null) {
      const obj = {};
            Object.assign(obj, item);
      newArr.push(obj);
    }
    arr.forEach(function (item2, index2) {
      if (item.id === item2.parentId) {
        const obj = {};
                Object.assign(obj, item2);
        item.children.push(obj);
      }
    })
  })
  return newArr[0];
}
function flattenArray(arr) {  //多维转一维
  var newArr = [];
  return (function fn(arr) {
    if (Object.prototype.toString.call(arr) === '[object Object]') {
      var att = arr.children && arr.children.length ? JSON.parse(JSON.stringify(arr.children)) : [];
      arr.children = [];
      newArr.push(arr);
      fn(att);
    } else {
      arr.forEach(function (item, index) {
        console.log(item.children.length, 66)
        if (item.children.length) {
          var att = JSON.parse(JSON.stringify(item.children));
          item.children = [];
          newArr.push(item);
          fn(att);
        } else {
          newArr.push(item);
        }
      })
    }
    return newArr;
  })(arr)
}
// 这两个函数有副作用，会对外层arr造成影响
console.log(multiArray(arr));
console.log(flattenArray(multiArray(arr)));

没有函数副作用的语法

function AssignToNew(a) {
  // 没有副作用的
  function toNew(target, source) {
    const keys = Object.keys(source);
    keys.forEach((key) => {
      if (typeof source[key] === 'object') {
        if (Array.isArray(source[key])) {
          target[key] = [];
          toNew(target[key], source[key]);
        } else if (source[key] === null) {
          target[key] = null;
        } else if (source[key] === undefined) {
          target[key] = undefined;
        } else {
          target[key] = {};
          toNew(target[key], source[key]);
        }
      } else {
        target[key] = source[key]
      }
    });
  }
  if (typeof a === 'object') {
    if (Array.isArray(a)) {
      obj = [];
    } else if (a === null) {
      obj = null;
    } else if (a === undefined) {
      obj = undefined;
    } else {
      obj = {};
    }
    toNew(obj, a)
  } else {
    obj = a;
  }
  return obj;
}
function multiArray(list) {        //一维转多维
  const arr = AssignToNew(list);
  const newArr = [];
  arr.forEach(function (item, index) {
    if (item.parentId === null) {
      const obj = {};
            Object.assign(obj, item);
      newArr.push(obj);
    }
    arr.forEach(function (item2, index2) {
      if (item.id === item2.parentId) {
        const obj = {};
                Object.assign(obj, item2);
        item.children.push(obj);
      }
    })
  })
  return newArr[0];
}
function flattenArray(list) {  //多维转一维
  const arr = AssignToNew(list);
  var newArr = [];
  return (function fn(arr) {
    if (Object.prototype.toString.call(arr) === '[object Object]') {
      var att = arr.children && arr.children.length ? JSON.parse(JSON.stringify(arr.children)) : [];
      arr.children = [];
      newArr.push(arr);
      fn(att);
    } else {
      arr.forEach(function (item, index) {
        console.log(item.children.length, 66)
        if (item.children.length) {
          var att = JSON.parse(JSON.stringify(item.children));
          item.children = [];
          newArr.push(item);
          fn(att);
        } else {
          newArr.push(item);
        }
      })
    }
    return newArr;
  })(arr)
}
console.log(multiArray(arr));
console.log(flattenArray(multiArray(arr)));

Array赋值的注意事项与循环方式的差异

var a = [];
a['1'] = 2;
a['3'] = 3;
a['b'] = 4;
console.log('a.length:', a.length);
for (let i = 0; i < a.length; i += 1) {
    console.log('for:', a[i]);
}
for (let i in a) {
    console.log('for in:', i, a[i]);
}
a.forEach((a) => {
    console.log('forEach:', a);
});
/*
VM3641:5 a.length: 4
VM3641:7 for: undefined
VM3641:7 for: 2
VM3641:7 for: undefined
VM3641:7 for: 3
VM3641:11 for in: 1 2
VM3641:11 for in: 3 3
VM3641:11 for in: b 4
VM3641:14 forEach: 2
VM3641:14 forEach: 3
*/