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JavaScript/First program
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Here is a single JavaScript statement, which creates a pop-up dialog saying "Hello World!":
alert("Hello World!");
For the browser to execute the statement, it must be placed inside a HTML <script> element. This element describes which part of the HTML code contains executable code. It will be described in further detail later.
<script>
alert("Hello World!");
</script>
The <script> element should then be nested inside the <head> element of an HTML document. Assuming the page is viewed in a browser that has JavaScript enabled, the browser will execute (carry out) the statement as the page is loading.
<!DOCTYPE html>
<html lang="en">
<head>
<title>Some Page</title>
<script>
alert("Hello World!");
</script>
</head>
<body>
<p>The content of the web page.</p>
</body>
</html>
This basic 'Hello World' program can then be used as a starting point for any new program that you need to create.
Exercises
1. Copy and paste the basic program into a file, and save it on your hard disk as 'exercise_1-1.html'. You can run it in different ways:
By going to the file with a file manager and opening it using a web browser.
By starting your browser and then opening the file from the menu.
By starting your browser and then specify the URL to 'exercise_1-1.html' with the file protocol. Please note that a) there are 3 slashes after 'file:' and b) replace 'temp' with the name of your directory.
Windows: file:///C:/temp/exercise_1-1.html (it's Windows syntax, nevertheless use slash instead of backslash)
Linux: file:///temp/exercise_1-1.html
What happens?
100% A dialog appears with the text: Hello World!
2. Save the above file as 'exercise_1-2.html'. Replace the double quotes in the line alert("Hello World!"); with single quotes, so it reads alert('Hello World!'); and save the result. If you open this file in the browser, what happens?
100% Nothing changes. A dialog appears with the text: Hello World! Double quotes and single quotes (apostrophes) are equivalents in JS.
it:JavaScript/Il nostro primo programma
pl:JavaScript/Składnia języka
zh:JavaScript/Hello World!