Weird behavior of javascript

Question

Please see attached screenshot. See pendingApp property of Object. when I am debugging in eclipse then pendingApp show array of object, Which is correct! But when I am JSON.stringify(object) then showing me empty Array.

Please let me know reason of this behavior. I think I am not aware with any Java-Script thought/concept ?? :P :)

When I will save this Object into DB then blank array of pendingApp will be stored !!

var pending_app = [];
var new_record = {"pendingApp" : [], "installedApp" :[] };
....SOME CODE+conditions HERE....
 pending_app[appId] = {'action' : action };
 new_record.pendingApp = pending_app;
// create app-config data
 return app_model.create(new_record); //will return promise object

Its just manually created javascript object and why not save it into DB ??? — Manish Trivedi, Aug 14 '15 at 09:22
Wait for your array to get filled. You use some asynchronous functions, don't you? — Bergi, Aug 14 '15 at 09:23
But see also https://stackoverflow.com/questions/23667086/why-is-my-variable-undefined-after-i-modify-it-inside-of-a-function. The question I've closed as a duplicate of explains what you are seeing in the debugger. — Bergi, Aug 14 '15 at 09:24
@ Bergi : I got your point but why is it not storing into my db ??? — Manish Trivedi, Aug 14 '15 at 09:31
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/86970/discussion-between-manish-trivedi-and-bergi). — Manish Trivedi, Aug 14 '15 at 09:34
@ManishTrivedi , the variable `appId ` might not be a number, then `JSON.stringify` return empty array repr (as I said in my first comment) — Hacketo, Aug 14 '15 at 09:45
@Hacketo : May be You are right!! its sting-stingrValue kind of value. But JSON.stringify print it always. ( According to my past logs) — Manish Trivedi, Aug 14 '15 at 09:48
When I will create ` var new_record = {"pendingApp" : {}, "installedApp" :{} }; ` then issue is resolved. — Manish Trivedi, Aug 14 '15 at 10:05
@Bergi I will, not able to do it right now, android app seem buggy — Hacketo, Aug 14 '15 at 10:23

Hacketo · Accepted Answer · 2015-08-14T11:43:29.183

It's not a weird behaviour but a common mistake of using an Array to store key-value data.

Short Answer : Use a literal Object to store these data

While you can add properties on every objects in Javascript, you cannot iterate over them with the default array mechanisms

for (var i = 0; i < array.length; i++){}
array.forEach();

Simple demonstration :

var array = [];
array["anId"] = 1;
array.length; // 0

array[4294967295] = 1; // Indice >= unsigned 32-bit Max Value
array.length; // 0
array[4294967295]; // 1

So JSON.stringify with the ECMAScript 5 Specification will use the Array mechanism to iterate over all items and will find nothing.

Unlike Objects that you can list properties with

Object.keys(array); // ["anId"]

Weird behavior of javascript

1 Answers1