Develop Reference

load inserted script, making sure that libraries load first [duplicate]

I'm creating a jquery plugin and I want to verify an external script is loaded. This is for an internal web app and I can keep the script name/location consistent(mysscript.js). This is also an ajaxy plugin that can be called on many times on the page.
If I can verify the script is not loaded I'll load it using:
jQuery.getScript()
How can I verify the script is loaded because I don't want the same script loaded on the page more than once? Is this something that I shouldn't need to worry about due to caching of the script?
Update:
I may not have control over who uses this plugin in our organization and may not be able to enforce that the script is not already on the page with or without a specific ID, but the script name will always be in the same place with the same name. I'm hoping I can use the name of the script to verify it's actually loaded.

If the script creates any variables or functions in the global space you can check for their existance:
External JS (in global scope) --
var myCustomFlag = true;
And to check if this has run:
if (typeof window.myCustomFlag == 'undefined') {
//the flag was not found, so the code has not run
$.getScript('<external JS>');
}
Update
You can check for the existence of the <script> tag in question by selecting all of the <script> elements and checking their src attributes:
//get the number of `<script>` elements that have the correct `src` attribute
var len = $('script').filter(function () {
return ($(this).attr('src') == '<external JS>');
}).length;
//if there are no scripts that match, the load it
if (len === 0) {
$.getScript('<external JS>');
}
Or you can just bake this .filter() functionality right into the selector:
var len = $('script[src="<external JS>"]').length;

Few too many answers on this one, but I feel it's worth adding this solution. It combines a few different answers.
Key points for me were
add an #id tag, so it's easy to find, and not duplicate
Use .onload() to wait until the script has finished loading before using it
mounted() {
// First check if the script already exists on the dom
// by searching for an id
let id = 'googleMaps'
if(document.getElementById(id) === null) {
let script = document.createElement('script')
script.setAttribute('src', 'https://maps.googleapis.com/maps/api/js?key=' + apiKey)
script.setAttribute('id', id)
document.body.appendChild(script)
// now wait for it to load...
script.onload = () => {
// script has loaded, you can now use it safely
alert('thank me later')
// ... do something with the newly loaded script
}
}
}

#jasper's answer is totally correct but with modern browsers, a standard Javascript solution could be:
function isScriptLoaded(src)
{
return Boolean(document.querySelector('script[src="' + src + '"]'));
}
UPDATE July 2021:
The accepted solutions above have changed & improved much over time. The scope of my previous answer above was only to detect if the script was inserted in the document to load (and not whether the script has actually finished loading).
To detect if the script has already loaded, I use the following method (in general):
Create a common library function to dynamically load all scripts.
Before loading, it uses the isScriptLoaded(src) function above to check whether the script has already been added (say, by another module).
I use something like the following loadScript() function to load the script that uses callback functions to inform the calling modules if the script finished loading successfully.
I also use additional logic to retry when script loading fails (in case of temporary network issues).
Retry is done by removing the <script> tag from the body and adding it again.
If it still fails to load after configured number of retries, the <script> tag is removed from the body.
I have removed that logic from the following code for simplicity. It should be easy to add.
/**
* Mark/store the script as fully loaded in a global variable.
* #param src URL of the script
*/
function markScriptFullyLoaded(src) {
window.scriptLoadMap[src] = true;
}
/**
* Returns true if the script has been added to the page
* #param src URL of the script
*/
function isScriptAdded(src) {
return Boolean(document.querySelector('script[src="' + src + '"]'));
}
/**
* Returns true if the script has been fully loaded
* #param src URL of the script
*/
function isScriptFullyLoaded(src) {
return src in window.scriptLoadMap && window.scriptLoadMap[src];
}
/**
* Load a script.
* #param src URL of the script
* #param onLoadCallback Callback function when the script is fully loaded
* #param onLoadErrorCallback Callback function when the script fails to load
* #param retryCount How many times retry laoding the script? (Not implimented here. Logic goes into js.onerror function)
*/
function loadScript(src, onLoadCallback, onLoadErrorCallback, retryCount) {
if (!src) return;
// Check if the script is already loaded
if ( isScriptAdded(src) )
{
// If script already loaded successfully, trigger the callback function
if (isScriptFullyLoaded(src)) onLoadCallback();
console.warn("Script already loaded. Skipping: ", src);
return;
}
// Loading the script...
const js = document.createElement('script');
js.setAttribute("async", "");
js.src = src;
js.onload = () => {
markScriptFullyLoaded(src)
// Optional callback on script load
if (onLoadCallback) onLoadCallback();
};
js.onerror = () => {
// Remove the script node (to be able to try again later)
const js2 = document.querySelector('script[src="' + src +'"]');
js2.parentNode.removeChild(js2);
// Optional callback on script load failure
if (onLoadErrorCallback) onLoadErrorCallback();
};
document.head.appendChild(js);
}

This was very simple now that I realize how to do it, thanks to all the answers for leading me to the solution. I had to abandon $.getScript() in order to specify the source of the script...sometimes doing things manually is best.
Solution
//great suggestion #Jasper
var len = $('script[src*="Javascript/MyScript.js"]').length;
if (len === 0) {
alert('script not loaded');
loadScript('Javascript/MyScript.js');
if ($('script[src*="Javascript/MyScript.js"]').length === 0) {
alert('still not loaded');
}
else {
alert('loaded now');
}
}
else {
alert('script loaded');
}
function loadScript(scriptLocationAndName) {
var head = document.getElementsByTagName('head')[0];
var script = document.createElement('script');
script.type = 'text/javascript';
script.src = scriptLocationAndName;
head.appendChild(script);
}

Create the script tag with a specific ID and then check if that ID exists?
Alternatively, loop through script tags checking for the script 'src' and make sure those are not already loaded with the same value as the one you want to avoid ?
Edit: following feedback that a code example would be useful:
(function(){
var desiredSource = 'https://sitename.com/js/script.js';
var scripts = document.getElementsByTagName('script');
var alreadyLoaded = false;
if(scripts.length){
for(var scriptIndex in scripts) {
if(!alreadyLoaded && desiredSource === scripts[scriptIndex].src) {
alreadyLoaded = true;
}
}
}
if(!alreadyLoaded){
// Run your code in this block?
}
})();
As mentioned in the comments (https://stackoverflow.com/users/1358777/alwin-kesler), this may be an alternative (not benchmarked):
(function(){
var desiredSource = 'https://sitename.com/js/script.js';
var scripts = document.getElementsByTagName('script');
var alreadyLoaded = false;
for(var scriptIndex in document.scripts) {
if(!alreadyLoaded && desiredSource === scripts[scriptIndex].src) {
alreadyLoaded = true;
}
}
if(!alreadyLoaded){
// Run your code in this block?
}
})();

Simply check if the global variable is available, if not check again. In order to prevent the maximum callstack being exceeded set a 100ms timeout on the check:
function check_script_loaded(glob_var) {
if(typeof(glob_var) !== 'undefined') {
// do your thing
} else {
setTimeout(function() {
check_script_loaded(glob_var)
}, 100)
}
}

Another way to check an external script is loaded or not, you can use data function of jquery and store a validation flag. Example as :
if(!$("body").data("google-map"))
{
console.log("no js");
$.getScript("https://maps.googleapis.com/maps/api/js?v=3.exp&sensor=false&callback=initilize",function(){
$("body").data("google-map",true);
},function(){
alert("error while loading script");
});
}
}
else
{
console.log("js already loaded");
}

I think it's better to use window.addEventListener('error') to capture the script load error and try to load it again.
It's useful when we load scripts from a CDN server. If we can't load script from the CDN, we can load it from our server.
window.addEventListener('error', function(e) {
if (e.target.nodeName === 'SCRIPT') {
var scriptTag = document.createElement('script');
scriptTag.src = e.target.src.replace('https://static.cdn.com/', '/our-server/static/');
document.head.appendChild(scriptTag);
}
}, true);

Merging several answers from above into an easy to use function
function GetScriptIfNotLoaded(scriptLocationAndName)
{
var len = $('script[src*="' + scriptLocationAndName +'"]').length;
//script already loaded!
if (len > 0)
return;
var head = document.getElementsByTagName('head')[0];
var script = document.createElement('script');
script.type = 'text/javascript';
script.src = scriptLocationAndName;
head.appendChild(script);
}

My idead is to listen the error log if there is an error on script loading.
const checkSegmentBlocked = (e) => {
if (e.target.nodeName === 'SCRIPT' && e.target.src.includes('analytics.min.js')) {
window.isSegmentBlocked = true;
e.target.removeEventListener(e.type, checkSegmentBlocked);
}
};
window.addEventListener('error', checkSegmentBlocked, true);

Some answers on this page are wrong. They check for the existence of the <script> tag - but that is not enough. That tells you that the tag was inserted into the DOM, not that the script is finished loading.
I assume from the question that there are two parts: the code that inserts the script, and the code that checks whether the script has loaded.
The code that dynamically inserts the script:
let tag = document.createElement('script');
tag.type = 'text/javascript';
tag.id = 'foo';
tag.src = 'https://cdn.example.com/foo.min.js';
tag.onload = () => tag.setAttribute('data-loaded', true); // magic sauce
document.body.appendChild(tag);
Some other code, that checks whether the script has loaded:
let script = document.getElementById('foo');
let isLoaded = script && script.getAttribute('data-loaded') === 'true';
console.log(isLoaded); // true
If the both of those things (inserting and checking) are in the same code block, then you could simplify the above:
tag.onload = () => console.log('loaded');

I found a quick tip before you start diving into code that might save a bit of time. Check devtools on the webpage and click on the network tab. The js scripts are shown if they are loaded as a 200 response from the server.

Bind click events on loaded ajax contents

I'm trying to load contents through ajax and then perform some click events on those contents.
So basically I want to first load the contents(Buttons in this case) and then apply some click events on them .
and hence I'm using callbacks for the purpose to load the contents first and then apply click events , but here I've a problem that the content is loaded after the callback function, and I don't want this, I want to load the content first then execute callback function, how to solve this problem?
To achieve this till now I've the following code.
I've following code in food.php:
<button onclick="findWeek(fun)">week</button> // fun is callback function passed to findWeek()
// findWeek function ------ AJAX request to load buttons ------:
function findWeek(fun)
{
var xhr = new XMLHttpRequest();
xhr.open("GET" ,"start.php",true);
xhr.send(null);
xhr.onreadystatechange = function(){
if( (xhr.readyState == '4') && ( xhr.status == '200') ){
document.getElementById("stats").innerHTML = xhr.responseText;
}
};
fun();
}
// fun function, is a callback function, and I'm assuming that this will come in action when the findWeek() function load the contents,meaning that it will load the required buttons from the page start.php inserting those buttons to the following div which is inside food.php.
<div id = 'stats'>
</div>
And after that I want to be able to click those loaded buttons then which are supposed to be there in the above div.
That's why I've fun() function something like following.
Note: I've defined class .dayBtns for the loaded buttons.
function fun(){
function myfunction(){
alert('just clicked the button');
}
var dayBtns = document.getElementsByClassName('dayBtns');
alert('contents should be loaded');
for(var i = 0; i < dayBtns.length; i++){
console.log(dayBtns[i]);
btns[i].addEventListener('click', myfunction);
}
}
The problem is the content is loading after the fun() function execution, how to restrict fun() function not to execute until the data is not loaded ?
Please help , thanks !

Just place fun() into onreadystatechange like so -
function findWeek(fun) {
var xhr = new XMLHttpRequest();
xhr.open("GET", "start.php", true);
xhr.send(null);
xhr.onreadystatechange = function() {
if ((xhr.readyState == '4') && (xhr.status == '200')) {
document.getElementById("stats").innerHTML = xhr.responseText;
fun();
}
};
}
The innerHTML is set after the function fun is called so there are no elements in the DOM at that point.
You also have a typo on the code here:
for(var i = 0; i < dayBtns.length; i++){
console.log(dayBtns[i]);
btns[i].addEventListener('click', myfunction);
}
Should be:
for(var i = 0; i < dayBtns.length; i++){
console.log(dayBtns[i]);
dayBtns[i].addEventListener('click', myfunction);
}
Notice dayBtns vs btns

Don't show page until content has fully loaded

I am creating a landing page which should exist in two languages. The texts that should be shown are in two JSON files, called accordingly "ru.json" and "en.json". When a user clicks on the "Change language" button, the following function is executed:
function changeLang(){
if (userLang == 'ru') {
userLang = 'en';
document.cookie = 'language=en';
}
else {
userLang = 'ru';
document.cookie = 'language=ru';
}
var translate = new Translate();
var attributeName = 'data-tag';
translate.init(attributeName, userLang);
translate.process();
}
Where Translate() is the following:
function Translate() {
//initialization
this.init = function(attribute, lng){
this.attribute = attribute;
if (lng !== 'en' && lng !== 'ru') {
this.lng = 'en'
}
else {
this.lng = lng;
}
};
//translate
this.process = function(){
_self = this;
var xrhFile = new XMLHttpRequest();
//load content data
xrhFile.open("GET", "./resources/js/"+this.lng+".json", false);
xrhFile.onreadystatechange = function ()
{
if(xrhFile.readyState === 4)
{
if(xrhFile.status === 200 || xrhFile.status == 0)
{
var LngObject = JSON.parse(xrhFile.responseText);
var allDom = document.getElementsByTagName("*");
for(var i =0; i < allDom.length; i++){
var elem = allDom[i];
var key = elem.getAttribute(_self.attribute);
if(key != null) {
elem.innerHTML = LngObject[key] ;
}
}
}
}
};
xrhFile.send();
}
Everything works fine, however, when a user opens the page for the first time, if his Internet connection is bad, he just sees the elements of the page without text. It is just 1-2 seconds, but still annoying.
The question is, is there any way to check the text has loaded and display the page elements only on this condition?

You can use $(document).ready() in this way
$(document).ready(function(){
//your code here;
})
You can use the JavaScript pure load event in this way
window.addEventListener('load', function () {
//your code right here;
}, false);
Source: Here

translate.process() is asynchronous code which needs to make a call to a server and wait for its response. What it means is that, when you call this function, it goes in the background to go do its own thing while the rest of the page continues loading. That is why the user sees the page while this function is still running.
One minimal way I can think around this is by adding this to your css files in the head tag.
body { display: none }
And then, under this.process function, after the for loop ends, add
document.body.style.display = 'block'

If you want to suppori IE8:
document.onreadystatechange = function () {
if (document.readyState == "interactive") {
// run some code.
}
}

Put the code you want to execute when the user initially loads the page in a DOMContentLoaded event handler like below:
document.addEventListener('DOMContentLoaded', function() {
console.log('Whereas code execution in here will be deffered until the initial HTML document has been completely loaded and parsed, without waiting for stylesheets, images, and subframes to finish loading.');
});
console.log('This will log immediatley');
It's important to note that DOMContentLoaded is different than the load event

Non-Fatal Javascript Error?

I think there's some kind of non-fatal bug in my script that's not allowing me to a/ debug the script with Firebug and b/ causes Firefox to constantly display a Connecting... (with the swirly) while I'm on the page. The script seems to run fine though.
Any ideas what could cause that?
<script type="text/javascript">
var xmlHttp;
var xmlDoc;
loadXMLFile();
function loadXMLFile()
{
xmlHttp = new window.XMLHttpRequest();
xmlHttp.open("GET", "myFile.xml", true);
xmlHttp.onreadystatechange = StateChange;
xmlHttp.send(null);
}
function StateChange()
{
if ( xmlHttp.readyState == 4 )
{
xmlDoc = xmlHttp.responseXML;
processXML();
}
}
function processXML()
{
var firstNames = xmlDoc.querySelectorAll("name");
if (firstNames == null)
{
document.write("Oh poo. Query selector returned null.");
}
else
{
for (var i = 0; i < firstNames.length; i++)
{
document.write(firstNames[i].textContent + "<br>");
}
}
}
</script>

All the code in your page is parsed, but not executed before the page is completed. This happens, since you're calling document.write() from onreadystatechange event handler function rather than parsing time.
In this case document.write() implicitly calls document.open(), which wipes all the code out of the page, and only what is left is the text written by document.write(). Also document is left open, which causes the browser being "busy". This can be avoided by using document.close(), but it won't prevent the original content to vanish.
You need to add an element to the body and then use some "real" DOM manipulation method. Something like this:
<div id="qResult"></div>
Then instead of document.write():
document.getElementById('qResult').innerHTML = WHAT_EVER_NEEDED

Dynamically loading Javascript files and load completion events

today I've been working on loading dynamic javascript code (files). The solution I use is :
function loadScript(scriptUrl) {
var head = document.getElementsByTagName("head")[0];
var script = document.createElement('script');
script.id = 'uploadScript';
script.type = 'text/javascript';
script.src = scriptUrl;
head.appendChild(script);
}
the problem with this is that I don't know how to make sure WHEN the script contents are executed. Since the script contains classes (well JS classes), I wrote down a function that is called through setTimeout and checks if those objects are defined. This is not flexible as it is not automatical. So? Are there ways to load those scripts and have a reliable notification on when they have been executed?

You can use jquery's getScript and pass a callback function.
$.getScript("test.js", function(){
alert("Script loaded and executed.");
});
See: jquery.

The easiest way short of a JS library is to make an XMLHttpRequest and eval() the return. Your "onload" would be when the data is returned, and "oninit" would be right after you've eval'd.
EDIT: If you want to sequence this, you can create an AssetLoader that takes an array of scripts and won't eval() a script until the one preceding it has been fetched and initialized.
EDIT 2: You can also use the script.onload stuff in the post referenced in the comments. The eval method has a slight advantage in that you can separate and control the load and execution portions of the script import.
EDIT 3: Here's an example. Consider a file called foo.js that contains the following:
function foo () {
alert('bar');
}
Then execute the following from another page in your browser:
function initScript (scriptString) {
window.eval(scriptString);
}
function getScript (url, loadCallback, initCallback, callbackScope) {
var req = new XMLHttpRequest();
req.open('GET', url);
req.onreadystatechange = function (e) {
if (req.readyState == 4) {
if (loadCallback) loadCallback.apply(callbackScope);
initScript.call(null, req.responseText);
if (initCallback) initCallback.apply(callbackScope);
}
}
req.send();
}
function fooScriptLoaded () {
alert('script loaded');
}
function fooScriptInitialized () {
alert('script initialized');
foo();
}
window.onload = function () {
getScript('foo.js', fooScriptLoaded, fooScriptInitialized, null);
}
You will see the alerts "script loaded", "script initialized", and "bar". Obviously the implementation of XMLHttpRequest here isn't complete and there are all sorts of things you can do for whatever scope you want to execute the script in, but this is the core of it.

You could use a counter variable, and a kind of callback:
var scriptsToLoad = 10;
var loadedScripts = 0;
//...
function increaseLoadCount() {
loadedScripts++;
if(loadedScripts == scriptsToLoad) {
alert("start here");
}
}
//script1-10.js
increaseLoadCount();
Maybe a bit nicer than with a timeout..