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1.3 JS 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 1: My First Web Page

<!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>

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 2: My First Web Page part 2

<!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 loaded, will delete all existing HTML:

Example 3: My First Web Page part 3

<!DOCTYPE html>
<html>
<body>

<h1>My First Web Page</h1>
<p>My first paragraph.</p>

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

</body>
</html>

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

Using window.alert()

You can use an alert box to display data:

Example 4: My First Web Page part 4

<!DOCTYPE html>
<html>
<body>

<h1>My First Web Page</h1>
<p>My first paragraph.</p>

<script>
window.alert(5 + 6);
</script>

</body>
</html>

You can skip the window keyword.
In JavaScript, the window object is the global scope object, that means that variables, properties, and methods by default belong to the window object. This also means that specifying the window keyword is optional:

Example 5: My First Web Page part 5

<!DOCTYPE html>
<html>
<body>

<h1>My First Web Page</h1>
<p>My first paragraph.</p>

<script>
alert(5 + 6);
</script>

</body>
</html>

Using console.log()

For debugging purposes, you can call the console.log() method in the browser to display data.

You will learn more about debugging in a later chapter.

Example 6: Activate Debugging

<!DOCTYPE html>
<html>
<body>

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

</body>
</html>

JavaScript Print

JavaScript does not have any print object or print methods.
You cannot access output devices from JavaScript.
The only exception is that you can call the window.print() method in the browser to print the content of the current window.

Example 7: The window.print() Method

<!DOCTYPE html>
<html>
<body>

<button onclick="window.print()">Print this page</button>

</body>
</html>

JavaScript Fundamentals course home
Module 1. Introduction to JavaScript

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Example files created in this module:
What Can JavaScript Do? part 1 What Can JavaScript Do? part 2 What Can JavaScript Do? part 3 What Can JavaScript Do? part 4 What Can JavaScript Do? part 5 What Can JavaScript Do? part 6 JavaScript in Body Demo JavaScript in Head Demo JavaScript in Body Demo External JavaScript External JavaScript part 1 External JavaScript part 2 External JavaScript part 3 My First Web Page part 1 My First Web Page part 2 My First Web Page part 3 My First Web Page part 4 My First Web Page part 5 Activate Debugging The window.print() Method JavaScript Statements part 1 JavaScript Statements part 2 JavaScript Statements part 3 JavaScript Statements part 4 JavaScript Statements part 5 JavaScript Statements part 6 JavaScript Numbers JavaScript Strings JavaScript Variables JavaScript Operators Assigning JavaScript Values JavaScript Expressions part 1 JavaScript Expressions part 2 JavaScript Expressions part 3 The var Keyword Creates Variables JavaScript Comments are NOT Executed JavaScript is Case Sensitive More JavaScript Comments JavaScript Comments part 2 JavaScript Comments part 3 JavaScript Comments part 4 JavaScript Comments part 5