How to get the instance of array length in Json array in JS

As a front-end page developer dealing with json data for the first time, I encountered 'js gets the length of the array in the json array'? I didn’t even know that json doesn’t have the .lengthattribute (I really want to ridicule myself). If a young man doesn’t work hard, he will be miserable! I used to seek help from my boyfriend, but recently I try to solve the problems myself.

The problem I encountered is this: In the page at the end of ***.jsp, the json data passed to me by the backend is an array, and then I need to get the second array. The length of the array in contacts, and then Baidu discovered that json does not have a .length attribute. How to do it?

Since the jsonobject does not have a length attribute, what should I do if I want to know its length?

 var jslength=0; 
 for(var js2 in json){
   jslength++;
 }

Write this code as a Method, just call it later:

function getJsonLength(jsonData){    
var jsonLength = 0;    
for(var item in jsonData){
       jsonLength++;
    }    
    return jsonLength;
}

But the above method can only get the length of the first layer array? ? ? Can't get the array length in the subarray? ? ? Then I played with myself and successfully obtained the length of the contacts array using the following method.

var _data =  ${contactJson};
function getJsonLength(jsonData){　　
var jsonLength = 0;　　
for(var item in jsonData){　　　　
if(item == 'contacts'){　　　　　　
for(var x in jsonData[item]){
　　　　　　　　jsonLength++;
　　　　　　}
　　　　}
　　}        
　　return jsonLength;
}var _contact_num = getJsonLength(_data);
$('#contactNum').text(_contact_num);　　

The final result:

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