run js in html code example

Example 1: hello world javascript

// Output Hello World! in the console.
console.log("Hello World!");

Example 2: javascript hello world

I would do something like this:

<script>
  alert("Hello World!");
</script>

Example 3: call js script in html

<script src="file1.js" type="text/javascript"></script> 
<script src="file2.js" type="text/javascript"></script>