javascript - js multidimensional array problem

Question

[
    {
        "id": 1,
        "name": "sys",
        "title": "系统设置",
        "type": 1,
        "status": 1,
        "condition": "",
        "pid": 0,
        "level": 0,
        "sort": 7,
        "icon": "fa-gear",
        "children": [
            {
                "id": 11,
                "name": "conf/lst",
                "title": "配置列表",
                "type": 1,
                "status": 1,
                "condition": "",
                "pid": 1,
                "level": 1,
                "sort": 50,
                "icon": null,
                "children": [
                    {
                        "id": 12,
                        "name": "conf/add",
                        "title": "添加配置",
                        "type": 1,
                        "status": 1,
                        "condition": "",
                        "pid": 11,
                        "level": 2,
                        "sort": 50,
                        "icon": null,
                        "children": []
                    },
                    {
                        "id": 13,
                        "name": "conf/del",
                        "title": "配置删除",
                        "type": 1,
                        "status": 1,
                        "condition": "",
                        "pid": 11,
                        "level": 2,
                        "sort": 50,
                        "icon": null,
                        "children": []
                    },
                    {
                        "id": 14,
                        "name": "conf/edit",
                        "title": "配置编辑",
                        "type": 1,
                        "status": 1,
                        "condition": "",
                        "pid": 11,
                        "level": 2,
                        "sort": 50,
                        "icon": null,
                        "children": []
                    }
                ]
            },
            {
                "id": 9,
                "name": "conf/conf",
                "title": "配置项",
                "type": 1,
                "status": 1,
                "condition": "",
                "pid": 1,
                "level": 1,
                "sort": 50,
                "icon": null,
                "children": []
            }
        ]
    },
    {
        "id": 15,
        "name": "admin",
        "title": "管理员",
        "type": 1,
        "status": 1,
        "condition": "",
        "pid": 0,
        "level": 0,
        "sort": 50,
        "icon": "fa-user",
        "children": [
            {
                "id": 16,
                "name": "admin/lst",
                "title": "管理员列表",
                "type": 1,
                "status": 1,
                "condition": "",
                "pid": 15,
                "level": 1,
                "sort": 50,
                "icon": null,
            },
            {
                "id": 27,
                "name": "authrule/lst",
                "title": "权限列表",
                "type": 1,
                "status": 1,
                "condition": "",
                "pid": 15,
                "level": 1,
                "sort": 50,
                "icon": null,
            },
            {
                "id": 30,
                "name": "authgroup/lst",
                "title": "用户组",
                "type": 1,
                "status": 1,
                "condition": "",
                "pid": 15,
                "level": 1,
                "sort": 50,
                "icon": null,
            }
        ]
    }
]

The json above is a multi-dimensional array. I want to use js for loop to output the array under children, but I don’t know why it cannot be output and no error is reported.

$.ajax({
    type: "get",
    url: "/admin/index/menu",
    async: true,
    dataType: 'json',
    success: function(res) {
        for(var i = 0; i < res.length; i++) {
            console.log(res[i].children);    //这个能输出
            for (var a=0;a<res[i].children;a++) {
                console.log(res[i].children[a]);    //这个不能输出,也没有报错
            }
        }
    }
})

Excuse me, what’s wrong?

Answer 1

$.ajax({
    type: "get",
    url: "/admin/index/menu",
    async: true,
    dataType: 'json',
    success: function(res) {
        for(var i = 0; i < res.length; i++) {
            console.log(res[i].children);
            for (var a = 0; a < res[i].children.length; a++) { // <-- 此处少了.length，数字和对象比较大小，结果为false，第二个条件一次也满足不了
                console.log(res[i].children[a]);
            }
        }
    }
}

Answer 2

a<res[i].children -> a<res[i].children.length

Answer 3

Although I’m late, I think I can add something else

Generally, I personally prefer to use foreach traversal, in JS (take the code in this example as an example)

res.forEach(r => {
    r.children.forEach(c => {
        // do something
    });
});

The arrow function of es6 is used above. If you want to write it in es5, just change it to function expression

Answer 4

This should be a recursion. It is recommended to understand the knowledge of recursion
Recursively traverse nodes