JavaScript Output

JavaScript Display Possibilities
JavaScript can "display" data in different ways:

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

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>

<h2>My First Web Page</h2>
<p>My First Paragraph</p>

<p id="demo"></p>
<script>

document.getElementById("demo").innerHTML = 5 + 6;
</script>
</body>
</html>

output

My First Web Page

My First Paragraph.
11

Changing the innerHTML property of an HTML element is a common way to display data in HTML.

Using document.write()

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

Example

<!DOCTYPE html>
<html>
<body>

<h2>My First Web Page</h2>
<p>My first paragraph.</p>

<script>
document.write(5 + 6);

</script>
</body>
</html>

output

My First Web Page

My first paragraph.
Never call document.write after the document has finished loading.
It will overwrite the whole document.

Using document.write() after an HTML document is loaded, will delete all existing HTML:

Example

<!DOCTYPE html>
<html>
<body>

<h2>My First Web Page</h2>
<p>My first paragraph.</p>

<button type="button" onclick="document.write(5 + 6)">Try it</button>

</body>
</html>

output

My First Web Page

My first paragraph.

The document.write() method should only be used for testing.

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(5 + 6);
</script>

</body>
</html>

output

My First Web Page

My first paragraph.

Using console.log()

For debugging purposes, you can use the console.log() method to display data.
You will learn more about debugging in a later chapter.

Example

<!DOCTYPE html>
<html>
<body>

<script>
 <h2>Activate debugging with F12</h2>

<p>Select "Console" in the debugger menu. Then click Run again.</p>

 
console.log(5 + 6);
</script>

</body>
</html>

output

Activate debugging with F12

Select "Console" in the debugger menu. Then click Run again.

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