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Javascript first script

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Knowing that javascript needs to be entered between <script> tags, is a start.
But there are a few other things you need to know before writing your first javascript:

  • Javascript lines end with a semicolon.
  • Always put the text within " ".
  • Javascript is case sensitive i.e. capital letters are different from lowercase letters.

Note : Incorrect capitalization is probably the most common source of error for javascript programmers on all levels!!

JavaScript does NOT have any built-in print or display functions.
It can "display" data in different ways:

- Writing into an alert box, using window.alert().
- Writing into the HTML output using document.write().
- Writing into an HTML element, using innerHTML.
- Writing into the browser console, using console.log().


Using window.alert()

You can use an alert box to display data:

Example

<!DOCTYPE html>
<html>
<body>
  <h1>My First Web Page</h1>
  <p>My first paragraph.</p>
  <script>
     window.alert(25);
  </script>
</body>
</html>

Using document.write()

For testing purposes, it is convenient to use document.write():

Example

<!DOCTYPE html>
<html>
<body>
   <h1>My First Web Page</h1>
   <p>My first paragraph.</p>
   <script>
        document.write(5 + 6);
   </script>
</body>
</html>

Using document.write() after an HTML document is fully loaded, will delete all existing HTML:
Note The document.write() method should be used only for testing.


Using innerHTML

To access an HTML element, JavaScript can use the document.getElementById(id) method.
The id attribute defines the HTML element. The innerHTML property defines the HTML content:

Example

<!DOCTYPE html>
<html>
<body>
  <h1>My First Web Page</h1>
  <p>My First Paragraph</p>
  <p id="demo"></p>
  <script>
        document.getElementById("demo").innerHTML = 5 + 6;
  </script>
</body>
</html>

Note To "display data" in HTML, (in most cases) you will set the value of an innerHTML property.


Using console.log()

In your browser, you can use the console.log() method to display data.
Activate the browser console with F12, and select "Console" in the menu.

Example

<!DOCTYPE html>
<html>
<body>
  <h1>My First Web Page</h1>
  <p>My first paragraph.</p>
  <script>
        console.log(5 + 6);
  </script>
</body>
</html>

 

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