JavaScript top or bottom - location sensitive? - javascript

Why does a linked JavaScript file sometimes not work when it is included at the top of the page and not at the bottom?
<script type="text/javascript" src"..."></script>

For example, if you want to manipulate DOM items, and those are not yet existing, it won't work. If the JavaScript file is included in the head, the body is not existing yet, but if you include it at the end of the body, those items are valid.
If you don't want to rely on this behaviour, you may define a callback, which is run, when the document is ready, i.e. when the whole of the DOM is loaded already.
This is what e.g. jQuery achieves with $(document).ready(function() {}), or more shortly $(function () {});. In vanilla JavaScript (using modern browsers, so IE9+) this can be achieved using
document.addEventListener("DOMContentLoaded", function() {
// code...
});

The best way to know why is it not working is by checking for JS error. Try to find out what errors you are getting when the script has been included at the top. As mentioned in the other response it can be because of DOM items. You can circumvent this issue by adding a "defer" tag to the script.
It can also be because of some JS object you are expecting to be present when this script runs. For example if your script tag is serving a JSONP request then you must have the function that processes the data. Otherwise you will get a "undefined function" error when the script runs.

JS code is executed instruction by instruction from top to bottom.
The code that calls a function needs to be under that functions definition.
This code works:
var func = function()
{
alert('it works');
};
func();
While this doesn't:
func();
var func = function()
{
alert('it works');
};
It throws an undefined error. The reason for this is that JS compiler is not aware of the func definition at the time it tries to call it.
Same goes for the JS files included in your HTML page. You can include them at the bottom as long as there are not dependencies in above sections, or, if they do not try to manipulate HTML code before page load.

Related

How can I clear references to Javascript functions that no longer exist?

I am working on a project that uses AJAX to download HTML, CSS and Javascript in one singe chunk of text then appends it to an element on the page. Here is the code:
_t.stage.empty();
_t.stage.html(DATA);
This works fine.
Here is the problem:
After adding the HTML to the stage, I call this function:
if(initApp != null && typeof(initApp) == "function") initApp();// Checks for initApp(). If exists, executes.
If I load a page that has this function, then load one that does NOT have this function, the function from the first page is executed. Here is some psuedo code to understand the results.
page 1:
This is a page.
<style>...</style>
<script> function initApp(){ alert("hello"); } </script>
When this page is run, an alert box with the text 'hello' is shown.
page 2: (no initApp() function)
This is page 2.
<style>...</style>
When the page is run, an alert box with the text 'hello' is shown.
Please note: These pages are loaded with AJAX and inserted into the HTML of an already loaded page.
It is not easy to tell exactly what you're trying to do, but if what you're trying to do is make it so that some other code that calls initApp() will cause nothing to happen when it calls that, then you can simply redefine the function to a do-nothing function like this:
initApp = function() {}
The most recent definition of a function takes precedence (e.g. replaces any prior definitions).
If your newly loaded code contains an implementation of initApp() that you don't want called the second time the script is loaded, then you're out of luck. You can't stop that. You will need to change the structure of your code so that the dynamically loaded code doesn't execute stuff you don't want to be executed. There are many different ways you could do that. For example, you could have a global boolean that keeps track of whether the init code has been called yet.
var initCalled = false;
function initApp() {
if (!initCalled) {
initCalled = true;
// rest of initialization code here
}
}
initApp(); // will only actually do anything the first time it's called
// even if it is loaded more than once
It appears from the comments that you seem to think that reloading a script tag with different code will somehow make code from the previous script go away. It will not. Once a function is loaded, it stays loaded unless it is redefined to mean something else or unless some code explicitly removed a property from an object. It does not matter how the code was loaded or whether it was in the core page or an external script file.
Javascript functions that no longer exist
This is a bad premise. The functions still exist, which is obvious from the fact that the second AJAX load ended up executing it. The fact that the <script> tags are replaced and no longer in the document doesn't undefine the function. It's like asking why is your TV still broken if the burglar that broke it is no longer there.
There are two basic things you can do:
a) Clear the function explicitly yourself:
if (initApp != null && typeof(initApp) == "function") {
initApp();
delete window.initApp;
}
b) Change the function name to be unique per AJAX page (or namespace the function with the same idea), probably tied to the name of the AJAX page, so you can invoke it in a more specific manner.

jquery-extension not executing in chrome app

I've encountered a strange problem with my chrome app i'm developing at the moment.
I want to use the "knob" extension to use it for a alarm-clock slider (to set the time)
This wouldn't be a really difficult matter, wouldn't it be for the restrictions and strange issues found while programming a chrome app.
In my index.html file I included the jquery library and the knob extension. And that is where the problems started. Somehow, my scripts only can use the Id's of elements that are above them. So when I include the tags between the tags, nothing executes, if I put them after the first tags they only work with the things that are in this div container. thats why I put the script just before the tag. That works well for "normal" javascript usability. But because I have a that referes to a jquery function (for the knob) the jquery library should be already loaded before the function gets executed (if not, it just doesn't work). I tried to get a workaround by using these posibilities:
document.onload=test();
function test(){
$(function() {
$(".dial").knob();
});
}
document.onload=test();
$(function test() {
$(".dial").knob();
});
}
document.onload=$(function() {
$(".dial").knob();
});
}
well.... It didn't work. I also tried window.onload, with the same reuslt. does someone have a solution? It would be of great help.
Thank you,
neissen
Try like this:
$(function() { //document ready function
function test(){ //inside the ready function
$(".dial").knob();
}
test(); // and call the function here
});
Your Problems:
jQuery may not be loaded but you used some vanilla JS to handle jQuery, which will cause errors.
May be related to global and local.
For the external scripts, functions are only fired locally, which means the global object which contains the html won't be able to be accessed.
To make a global function and a jQuery library - JS:
if ("undefined" === typeof jQuery) {throw new Error("This library requires jQuery"); }
$(function() {
window.myFunction = function() {
doSomething();
}
doSomething();
})
Works calling from HTML - HTML:
<script>
$(function() {
doSomething();
})
</script>
Above is the safest way to approach a jQuery library. The $(function() {... part means exactly the same as $( document ).ready(function() {..., execute if loaded and ready, ensures the browser knows how to deal with all the functions used.

Javascript - Getting a script's own souce code from within the script

So I have a problem where I need to get the entirety of a script's source code as a string (whether it's an inline script or a script that uses the src attribute) from within the script itself, but without knowing beforehand exactly what script it is. I can't just do an XHR for that reason. Several scripts on the page will listen for events, and when they handle them they will also run code that identifies the script's own source code. How should I do this?
function identifySelf() {
// Some code here
return mysource; // should return the source code of the script in which this function is defined
}
You will need to identify each script tag with an id. Then
var script=document.getElementById("my_id");
var content=script.innerHTML || "";
if(content==null || content==""){
var src=script.src;
// ajax in the script, I'm not going to write the code because I'm on mobile
// I'm assuming you can write it yourself
}
EDIT: sorry, I didn't read your question fully. If you need to return the content right away, you will need to ajax in scripts' content on page load, and the return cached content. There is no way to just get the innerHTML like that.

Javascript executing when jQuery not ready

The following scenario is a problem I am having. I came to the conclusion that jQuery must not be ready when Javascript is executing by observing this scenario.
Scenario:
I have a Java application which injects Javascript script tags into the currently loaded DOM page. The following Java code runs inline Javascript which inserts jquery.js and myCode.js. myCode.js holds my Javascript codes.
browser.executeJavaScript("var head= document.getElementsByTagName('head')[0];" +
"var script= document.createElement('script');script.type= 'text/javascript';script.src= 'jquery.js';head.appendChild(script);" +
"var script4= document.createElement('script');script4.type= 'text/javascript';script4.src= 'http://myCode.js';head.appendChild(script4);");
In this Java application, I also have a buttonListener that fires a function in myCode.js in ActionPerformed();
executedJS = browser.executeJavaScript("replaceAllLinks()");
The problem that is encountered is nullPointerException at the above line when button is clicked. Accomodating for null case results in endless loop without any changes.
while(executedJS == null) browser.executeJavaScript("replaceAllLinks()");
The cause of the problem was pinpointed down to when jQuery functions, methods are present inside replaceAllLinks(); javascript function. when jQuery, methods were absent, no problems could be observed. There was not one instance of nullPointerException raised.
The only possible underlying issue would be that somehow jQuery library is not fully loaded while replaceAllLinks(); is being executed. If jQuery methods and functions were not in use, it doesn't matter and everything runs okay.
My question is then, how can I make sure that jQuery is fully loaded and available for use?
Every script relying on jQuery should be contained inside a DOM ready function. Such a function normally takes this form:
$(document).ready(function() {
/* code here */
});
and a shortcut to achieve the same thing would be:
$(function() {
/* code here */
});
Here's the documentation for further information on the ready method:
http://api.jquery.com/ready/
Declare some global variable at the end jquery.js, e.g.
window.jQueryIsLoaded=true;
and check this variable before using jQuery.
<edit>Forget this, see Salman A's comment below, should be the right answer.</edit>

Turn a .htc file's contents into 'regular' Javascript

I have a .htc file whose behaviour is attached to a div in my page (div#test). Within the file, there is a tag at the top, setting up the behaviour:
<PUBLIC:ATTACH EVENT="ondocumentready" FOR="element" ONEVENT="function1()" />
And throughout the file, there are calls to 'element', & this.element - which I presume are then referring to this div#test.
If I wanted to take the JS from this file, would it be possible to put into the main .html page? I've tried to make calls to the function on document load, but can't get my syntax correct.
I'm trying:
document.getElementById.('test').attachEvent(onlonad, function1());
Would appreciate any pointers, if I'm doing something basic wrong, or if anyone can tell me why doing it at all would be a bad idea! =)
You have a dot in the wrong place, you're passing an undefined variable to the function and you're calling function1() instead of passing it:
document.getElementById.('test').attachEvent(onlonad, function1());
// ^ this ^ ^ ^ and these ^^
Correct syntax would be
document.getElementById('test').attachEvent("onload", function1);
Also note that only a few elements support the onload event - images, scripts and the body (which maps to window.onload).
If you want to make calls on document load, then it's awkward in IE because it doesn't support the document ready event that other browsers support. There are ways around this, or you can use the window.onload event:
window.onload = function () {
// Code to execute when the window is loaded here
}

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