JavaScript Output

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

<!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

<!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

<!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

<!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

<!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

<!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

<!DOCTYPE html>
<html>
<body>

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

</body>
</html>

Comments

JavaScript Const

The const keyword was introduced in ES6 (2015) . Variables defined with const cannot be Redeclared. Variables defined with const cannot be Reassigned. Variables defined with const have Block Scope. Cannot be Reassigned A const variable cannot be reassigned : Example const PI = 3.141592653589793 ; PI = 3.14 ; // This will give an error PI = PI + 10 ; // This will also give an error Must be Assigned JavaScript const variables must be assigned a value when they are declared: Correct const PI = 3.14159265359 ; Incorrect const PI; PI = 3.1 4159 265359 ; When to use JavaScript const? As a general rule, always declare a variable with const unless you know that the value will change. Use const when you declare: A new Array A new Object A new Function A new RegExp Constant Objects and Arrays The keyword const is a little misleading. It does not define a constant value. It defines a constant reference to a value.

JavaScript Introduction

This page contains some examples of what JavaScript can do. JavaScript Can Change HTML Content One of many JavaScript HTML methods is getElementById() . The example below "finds" an HTML element (with id="demo"), and changes the element content (innerHTML) to "Hello JavaScript": Example document. getElementById ( "demo" ). innerHTML = "Hello JavaScript" ; JavaScript accepts both double and single quotes: Example document. getElementById ( 'demo' ). innerHTML = 'Hello JavaScript' ; JavaScript Can Change HTML Attribute Values In this example JavaScript changes the value of the src (source) attribute of an <img> tag: JavaScript Can Change HTML Styles (CSS) Changing the style of an HTML element, is a variant of changing an HTML attribute: Example document. getElementById ( "demo" ). style . fontSize = "35px" ; JavaScript Can Hide HTML Elements Hidin

JavaScript Let

The let keyword was introduced in ES6 (2015) . Variables defined with let cannot be Redeclared. Variables defined with let must be Declared before use. Variables defined with let have Block Scope. Cannot be Redeclared Variables defined with let cannot be redeclared . You cannot accidentally redeclare a variable. With let you can not do this: Example let x = "John Doe" ; let x = 0 ; // SyntaxError: 'x' has already been declared With var you can: Example var x = "John Doe" ; var x = 0 ; Block Scope Before ES6 (2015), JavaScript had only Global Scope and Function Scope . ES6 introduced two important new JavaScript keywords: let and const . These two keywords provide Block Scope in JavaScript. Variables declared inside a { } block cannot be accessed from outside the block: Example { let x = 2 ; } // x can NOT be used here Variables declared with the var keyword can NOT have block sco

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