javascript - Declaring an array to add a value produces a value of type undefined

Question

JS: Use objects to obtain successful values, use arrays, and there will be values ​​of type undefined

var bodydata = $("body").data(); 
// console.log(bodydata);
bodyDatas = {};/*使用了个全局变量bodyDatas*/
_.forIn(bodydata,function(value,key){
    if(typeof value == "string"){
        value = parseLodash(n);
    }
    if(value[0] == "200"){
        var keyarray = key.split('_');
        var key0 = keyarray[0];
        var key1 = keyarray[1];
        var key2 = keyarray[2];
        _.forIn(_.keys(value[2]), function(v,k) {
            if (!bodyDatas[key0]) {
                bodyDatas[key0] = {};
            }
            if (!bodyDatas[key0][key1]) {
                bodyDatas[key0][key1] = {};
            }   
            if (!bodyDatas[key0][key1][key2]) {
                bodyDatas[key0][key1][key2] = {};
            }
            if (!bodyDatas[key0][key1][key2][v]) {
                bodyDatas[key0][key1][key2][v] = value[2][v];
            }
        });        
    }else{
        console.log("取到数据头部:"+n[0]); 
    }
})

The above can obtain the correct results.

var bodydata = $("body").data(); 
bodyDatas = new Array();/*使用了个全局变量bodyDatas*/

_.forIn(bodydata,function(value,key){
    if(typeof value == "string"){
        value = parseLodash(n);
    }
    if(value[0] == "200"){
        var keyarray = key.split('_');
        var key0 = keyarray[0];
        var key1 = keyarray[1];
        var key2 = keyarray[2];
        _.forIn(_.keys(value[2]), function(v,k) {
            if (!bodyDatas[key0]) {
                bodyDatas[key0] = [];
            }
            if (!bodyDatas[key0][key1]) {
                bodyDatas[key0][key1] = [];
            }
        });        
    }else{
        console.log("取到数据头部:"+n[0]); 
    }
})              
console.log(bodyDatas["mx"]);

The above code uses an array, and there will be a value with the default type undefined
Although I solved this problem by replacing the object with the array, I still have questions in my mind. : Why when using an array, a value of type undefined is added, but this value does not exist. My loop is indeed only executed once, so maybe there is something wrong with the way I declare the array. Is there a default value in it? How should it be declared?

Answer 1

You can refer to this question
/q/10...