In a web page, when JavaScript starts to run? For example,
HTML section 1
Javascript section 1
Event handler routine 1
HTML section 2
Javascript section 2
Event handler routune 2
Is this running order correct?
Will no event handler routine run before JavaScript section 2 is finished?
No, js is executed in place where it is defined - so in below example you cannot use div defined in section 2 html (id='sec2') in js script defined before it (you will see error). However if you use html-inline event handlers then you can use js function defined after html element definition
<body>
<div id='sec1' onclick="show('sec1')">HTML section 1 (click me)</div>
<script>
console.log('Javascript section 1');
console.log('sec1 html:', sec1);
console.log('sec2 html:', sec2);
</script>
<div id='sec2' onclick="show('sec2')"> HTML section 2 (click me)</div>
<script>
console.log('Javascript section 2');
console.log('sec1 html:', sec1);
console.log('sec2 html:', sec2);
function show(msg) { alert(msg) };
</script>
</body>In a web page, when JavaScript starts to run?
Well, it depends on where javascript is written. It is executed as soon as it is downloaded (from a file) or as soon as it is read by browser if inline in html.
If your javascript is inside some event like window.onload or $(document).load(...) then it will be executed after page is rendered.
Event handlers are executed as a response to an event. So, there is no specific order here. As soon as the event occurs, event handler is fired.
Consider following example.
<html>
<body>
<h1 id="first">
First
</h1>
<script type="text/javascript" lazyload>
var first = document.getElementById('first'); // logs element
var second = document.getElementById('second'); //logs null
console.log(first);
console.log(second);
</script>
<h1 id="second">
Second
</h1>
<script type="text/javascript" lazyload>
var first = document.getElementById('first'); //logs element
var second = document.getElementById('second'); //logs element
console.log(first);
console.log(second);
</script>
</body>
</html>
Above example demonstrates that #first was rendered before js block 1 executed but #second was not rendered by this time.