Javascript方法和局部变量

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我一直在搜索互联网,找不到正确的搜索词,所以我最终在这里再次询问Javascript中的所有老年人.
以下代码取自答案on these question
我真的不明白方法或功能是如何工作的.

var list = [{id: 1,title: 'home',parent: null},{id: 2,title: 'about',parent: null},{id: 3,title: 'team',parent: 2},{id: 4,title: 'company',parent: 2} ];
function treeify(list) {
     var treeList = [];
     var lookup = {};
     list.forEach(function(obj) {
          obj['children'] = [];
          lookup[obj['id']] = obj;
     });
     console.log(lookup); // [problem number 1]

     list.forEach(function(obj) {
          if (obj['parent'] != null) {
               lookup[obj['parent']]['children'].push(obj);
          } else {
               treeList.push(obj);
          }
     });

     console.log(treeList); // [problem number 2]
};

treeify(list);

关于问题1：

It resulted an object that had already a children on each parent which is supposedly I think that, parent should have an empty array children at that moment. How does it work? Please enlightened me.

关于问题2

treeList already formed the hierarchy tree. How it happen? it didn’t even push the lookup variable to the treeList variable? It only pushes the obj with a parent that is equal to null (which is the root parent).

希望得到你的答案.
如果你知道任何可以帮助我理解javascript的博客,文章等,请不要犹豫,在你的答案中添加链接.
很多谢谢！

解决方法:

var list = [
     {id: 1,title: 'home',parent: null},
     {id: 2,title: 'about',parent: null},
     {id: 3,title: 'team',parent: 2},
     {id: 4,title: 'company',parent: 2} 
];

treeify(list);


function treeify(list) {

     var treeList = []; //local array
     var lookup = {}; //local object

     // iterate over each element in list array
     list.forEach(function(obj) {

          // add a children property to each element in the array
          // in this case the children property is an array
          obj['children'] = [];

          // obj['id'] returns 1,2,3,4
          // lookup is an object so we use the id of each element in the list array as a key
          // first iteration adds key : 1 and value {id:1, title: 'home', parent: null, children: [] }
          // second iteration adds key : 2 and value {id:2, title: 'about', parent: null, children: [] }
          // ...
          lookup[obj['id']] = obj;
     });

     /*
          console.log(lookup) should output
          {
               1: {id: 1,title: 'home', parent: null, children: []},
               2: {id: 2,title: 'about', parent: null, children: []},
               3: {id: 3,title: 'team', parent: 2, children: []},
               4: {id: 4,title: 'company', parent: 2, children: []} 
          }

          however, if you run the code the lookup object gets modifyed in 
          the lines below (lookup[obj['parent']]['children'].push(obj);),
          therefore, some items in the lookup object will have children elements in its child array
     */
     console.log(lookup); // [problem number 1]

     list.forEach(function(obj) {
          if (obj['parent'] != null) {
               // this line modifyes the lookup object at runtime
               // obj has a parent, so we add it to the corresponding parents children array using the id
               lookup[obj['parent']]['children'].push(obj);
          } else {
               //only adds an element to the treelist array if its a parent element
               treeList.push(obj);
          }
     });

     console.log(treeList);
};

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