Develop Reference

How does render() actually execute?

I'm working on a project and I have created an interesting challenge for myself.
render() {
const sysObjs = this.state.systemObjects;
const jsonPackage = sysObjs[0];
if(typeof jsonPackage === 'undefined'){
console.log("undefined!!!")
} else {
//let jsonData = JSON.parse(jsonPackage.replaceAll("\\\\", "\\"));
console.log(jsonPackage.replaceAll("\\\\", "\\"));
}
console.log(jsonPackage.replaceAll("\\\\", "\\"));
If I run that it will fail and it will fail on the last line of the snippet and it will fail because from the perspective of that line of code jsonPackage is undefined and we get a "cannot read property 'replaceAll' of undefined. That makes sense to me as doing any sort of operation on something that is undefined would fairly difficult. The part that is confusing me though is that if I remove that last line then the code will execute. That's confusing to me because that console.log is also there within the if-else block so I would imagine that both should throw errors but they don't. In fact if I run that after removing the last line I will actually get both sections of the if-else block to log to the console and that tells me that this is executing in a way that I don't understand. Can anyone explain what's happening for me? At the root here something is doing something such that the const jsonPackage is not populated immediately when it is set so I'm curious how react actually handles something like that.
Lastly, I commented out that line because that line throws "Unhandled Rejection(Error) A cross-origin error was thrown." I'm also not too sure what that actually means either as it wasn't covered in the course I took.

render() is called each time your component's state updates. It can also get called in a number of other circumstances, but I think that's the relevant one for this situation.
You're not showing all of your code, but there is probably something running on mount or something similar that updates this.state.systemObjects.
On the first render of the component, this.state.systemObjects is undefined. So, if you have your console.log(jsonPackage.replaceAll("\\\\", "\\")); line in at the end of your render, it will fail because it can't do an operation on undefined. However, with it commented out, it runs fine, because it goes through the if(typeof jsonPackage === 'undefined'){ section of your if clause and not the else clause, where an operation is done on jsonPackage.
Then, once systemObjects is updated, render runs again. On that pass, systemObjects now is not undefined, so the else clause gets run -- since you don't have an undefined value any more, that else clause runs fine.
Your cross-origin error is not caused by any code you've shown -- it's probably caused by an API request somewhere. There are plenty of answers on SO about what can cause those.

I think you answered your own question. This is happening because the first time your code is executing, jsonPackage is not defined. This is why both blocks of your if statement are logging, because during the first pass though it is undefined and during the second it is, which will cause the else block to fire.
And this is why your code is breaking, during the first pass through of render, jsonPackage is not defined which will throw an error.
Its not really possible to explain why it's not defined the first time around without seeing the rest of your code. As a rule of thumb however, you should generally never assume that a React state value will always be defined and add checks accordingly.

Javascript structured error handling

I am developing my first Javascript app and I am trying to go object oriented.
There is a basic closure that returns my primary object and every function I invoke rests in that object. Some pseudo code would look like this:
primary = (function(){
var object = {
doSomething = function(){};
},
return {intance:function(return object)}
});
//invocation
primary.instance().doSomething();
What I am trying to achieve is to attach an error handler function to my object, so that whenever there is an internal error, it is cought, and I don't have to wrap every function call in a try catch block.
I tried object.onerrorbut the error went on to window object. Maybe I am getting the concept wrong. I tried searching on Github for some simpler framework that includes structured error handling, but no luck. I am pretty familiar with this in PHP, but I haven't done this so far in Javascript. Can somebody show me an example how it is done right?
EDIT: I know that structured error handling goes further, I am just trying to get a root handler, so that no errors / exceptions can pass on to the window object

Dealing with the error event without a try catch block will halt the execution of your script (except for any asynchronous functions that have already been called).
You can suppress (non-ajax, non-syntax) errors by capturing them on document.body or a more specific object, and stop them being thrown to the user (or reaching the window object) by using e.preventDefault() or return false, and send them to a global/object handler (to inspect or log) by passing the event object as an argument - but any of those options will stop your script execution beyond the point of error. That's the main benefit of a try catch block, and as far as I know there is no way around that.

Firebug's console.log: debugging asynchronous variables

I am trying to debug some javascript code. The variable I want to debug is a JS object of objects, loaded asynchronously (ajax callback). I am debugging it outside the callback.
I am pretty sure that the problem is a race-condition problem, but I want to be sure (and it is not my question).
The problem I find when debugging is that console.log gives me an information which does not make sense with what the code is doing. Therefore, either I am missing something in the code, or the result I see in the console is the not a snapshot of the state of the variable when I runned console.log.
If it turns out to be the later, how can i get a snapshot of the state of an asynchronously loaded JS object?
This is a simplified example of the code:
//This call takes a while to invoke the callback
$.get("url",function(data){
//Process data
globalVariable = data; //JSON (Object of objects)
}
//Couple lines afterwards
/* This console.log shows (in Firebug's console) "Object { }" and when I click it,
I can see the object with all its fields filled (oher objects)
*/
console.log(globalVariable);
for(var e in globalVariable){
//Does not enter here, meaning that the object is empty
}

console.log is itself asynchronous and it shows references rather than snapshots, sometimes.
If you log an object you will get a nice clickable interface where you can inspect the object. If the object changes you will see those changes reflected there.
If you want a real snapshot you're going to have to do some serialization and serializing objects is not trivial. If your objects just are data and have no functions or references you can use JSON.stringify(obj, undefined "\t").
A smarter way is to pause your asynchronous events so you can inspect the latest state of the object.

Write it like this instead. This way globalVariable will have the data before you act upon it.
$.get("url",function(data){
//Process data
globalVariable = data; //JSON (Object of objects)
for(var e in globalVariable){
//this will run
}
}

gridcomplete function always running?

I'm using jqgrid with grails and need to select some objects. This works for one given object. My problem is that sometimes, calling function has no id. So there is nothing to do after grid completion. And I'm not able to do that even after trying if (false)... In all case, grid.jqgrid("setSelection... is executed and I get the message
java.lang.NullPointerException
Cannot get property 'id' on null object
which is true. But why is this line executed ???
gridComplete: function() {
if (false) {
grid.jqGrid("setSelection",'${cableFocus.id}',true);
}
return;
I have a second question : How can I use gridComplete to select several rows instead of one ?
Thanks very much for any help,
Olivier

The problem is not with gridComplete or your if statement (which is javascript). As you are getting a java error i'm guessing ${cableFocus.id} is placeholder (which will always get exectuted)
So the javascript is not running but your java code is still attempting to run and throwing the error

Unable to access attribute after fetch

I am testing some code in my web console (using coffescript)
#user = new Onethingaday.Models.User()
#user.url= "/users/#{current_user.get('nickname')}.json?id_type=nickname"
#user.fetch()
console.log(#user)
console.log(#user.get("facebook_id"))
The console.log(#user) line shows the following:
However, console.log(#user.get("facebook_id")) shows undefined.
I see that user has the facebook_id attribute. How do I retrieve the value of it?

This looks like a timing issue, albeit an odd one. fetch is asynchronous; you need to wait until its success callback is called before trying to access any attributes.
Try this:
#user.fetch success: =>
console.log(#user)
console.log(#user.get("facebook_id"))
It's confusing that the first console.log would show user.attributes.facebook_id existing and the second wouldn't, but console.log is itself asynchronous in Webkit's implementation, so what's going on is that the #user.get call is being resolved immediately (synchronously), whereas the object inspection in the first console.log is resolving later—after the fetch completed.

It looks like facebook_id is a property of the attributes property of the #user object. If that's the case, I'd think the following would work:
#user.attributes.facebook_id