This section provides a quick description of the 'typeof' operator and data types supported in JavaScript. A tutorial example is provided to list all data types number, string, boolean, object, function, and undefined.
In the previous section, we learned how to use the "instanceof" operator.
But JavaScript offers another operator called "typeof", which works like "instanceof".
Here is a comparison of them:
1. "object_name instanceof constructor_name" returns true if the specified object
is an instance of the specified type.
2. "typeof expression" returns the data type name of the specified expression.
Remember that JavaScript supports several data types: "number", "string", "boolean", "object", "function", and "undefined".
To show you those data types and how to use "typeof", I wrote this tutorial example:
<html>
<!-- Prototype_typeof_Operator.html
Copyright (c) 2008 by Dr. Herong Yang, http://www.herongyang.com/
-->
<head>
<title>"typeof" Operator</title>
</head>
<body>
<pre>
<script type="text/javascript">
// Creating an object of "Book"
var myBook = new Book("JavaScript Tutorials", "Herong Yang");
// Show different data types
showDatatype(9, "9");
showDatatype(3.14, "3.14");
showDatatype("Hello", "\"Hello\"");
showDatatype(true, "true");
showDatatype(undefined, "undefined");
showDatatype(null, "null");
showDatatype(myBook, "myBook");
showDatatype(Book, "Book");
function showDatatype(x, name) {
document.writeln(name+": "+(typeof x));
}
function Book(title, author) {
this.title = title;
this.author = author;
}
</script>
</pre>
</body>
</html>
Here is the output of this tutorial example. Note that the data type of "null" is "object".
9: number
3.14: number
"Hello": string
true: boolean
undefined: undefined
null: object
myBook: object
Book: function
