Live Demo: JavaScript typeof Operator

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="utf-8">
 <title>JavaScript typeof Operator</title>
</head>
<body>
 <script>
 // Numbers
 document.write(typeof 15 + " "); // Prints: "number"
 document.write(typeof 42.7 + " "); // Prints: "number"
 document.write(typeof 2.5e-4 + " "); // Prints: "number"
 document.write(typeof Infinity + " "); // Prints: "number"
 document.write(typeof NaN + " "); // Prints: "number". Despite being "Not-A-Number"
 
 // Strings
 document.write(typeof '' + " "); // Prints: "string"
 document.write(typeof 'hello' + " "); // Prints: "string"
 document.write(typeof '12' + " "); // Prints: "string". Number within quotes is document.write(typeof string
 
 // Booleans
 document.write(typeof true + " "); // Prints: "boolean"
 document.write(typeof false + " "); // Prints: "boolean"
 
 // Undefined
 document.write(typeof undefined + " "); // Prints: "undefined"
 document.write(typeof undeclaredVariable + " "); // Prints: "undefined"
 
 // Null
 document.write(typeof Null + " "); // Prints: "object"
 
 // Objects
 document.write(typeof {name: "John", age: 18} + " "); // Prints: "object"
 
 // Arrays
 document.write(typeof [1, 2, 4] + " "); // Prints: "object"
 
 // Functions
 document.write(typeof function(){}); // Prints: "function"
 </script>
</body>
</html>