How to call a JavaScript function when the external JavaScript file is referenced at the end of the HTML body?
Question
I know that when you want to invoke a JavaScript function inside a HTML body section you can do it by putting <script> someFunction(); </script> <script> someFunction(); </script> inside your body tag, here is an example, I have HTML like this:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<script type="text/javascript"
src="/Script.js"></script>
</head>
<body>
<script>
showAlert();
</script>
</body>
</html>
And the javascript file like this:
function showAlert(){
alert("This is an alert!");
}
This works fine but if I put the JavaScript file reference at the end of the body like this:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
</head>
<body>
<script>
showAlert();
</script>
<script type="text/javascript"
src="/Script.js"></script>
</body>
</html>
the function showAlert() is no longer being invoked. Can you please answer these 2 questions:
- Why is
showAlert()not invoked in the second scenario? - How (if possible) to invoke a function from a JavaScript file when it is referenced in the end of the body?
The reason why I'm asking is because I know that it is a good practice to refer your JavaScript files in the end of the body instead of the head, so that the page will be rendered first before loading all the JavaScript code.
5 answers
solution1
4 2016-12-18 16:35:27
1) The scripts are loaded linearly. Since the script has not yet been loaded, the function is undefined. (This is in contrast to function hoisting within a script.)
2) Simply wait till the page loads.
window.onload = function(){
showAlert();
}
(Simply doing window.onload = showAlert won't work because of reason #1. Here you delay evaluation until such time that the function will exist.)
solution2
0 2016-12-18 16:36:56
Javascript processes in the order given. You are trying to call showAlert before showAlert have been defined. Change to:
<body>
<script type="text/javascript"
src="/Script.js">
</script>
<script>
showAlert();
</script>
</body>
and all should work as intended.
solution3
0 ACCPTED 2016-12-18 16:40:57
Assuming you want to run showAlert() immediately when the page has loaded, try adding an onload() event handler to call showAlert rather than just calling it as the script loads. This can be done a few ways:
<body onload="showAlert();">
or define the window onload event programatically where your current function all is made in the html
window.onload = new function() {showAlert();}
or (and I think this is the preferred way because it won't cancel out other event handlers bound to the onload event)
window.addEventListener("load", showAlert);
solution4
0 2016-12-18 16:47:58
By default, scripts run sequentially. Your code doesn't work because showAlert() runs before loading Script.js , so at that point the function showAlert is not defined yet.
To make it work, you must delay the showAlert call.
The load event has already been mentioned in other answers, but it will wait for all resources (like images) to load. So listening to the DOMContentLoad event is usually better, the function will be called sooner.
<script> document.addEventListener('DOMContentLoaded', function() { showAlert(); }); </script> <script src="data:text/javascript, function showAlert() { console.log('Hello!') } "></script>
solution5
0 2016-12-18 17:03:21
The reason for your script isn't working is the way how a webpage is parsed.. From top to bottom ..Here is some link (would help to know why script added at bottom).
1) in your First case the browser loaded script when it parsed the page and when you called it in body it was available so it got invoked.
2) in Second scenario (My be typo) You have placed the call to function before loading the script that contain your function. so during page parsing browser wont find it and continue to next line where script containing function is loaded which has no effect for now as it already parsed the call.
If you still want to follow the second scenario you have to trigger the function call (after ensuring all resources being loaded ie Your script). so you can use window.load=<your function call> or in case of jQuery place it inside
$(document).ready(function(){
//call here
});
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