CSCI 121 - Lecture 11 - JavaScript Variables, Data Types and Objects* (continued)

Topics Covered

JavaScript Data Types

JavaScript has six (6) different data types:

Number
String
Boolean
Object
Undefined
Null

Numbers may be integers or floating point or the special values NaN and Infinity.

var x1 = 34.00;     // Written with decimals
var x2 = 34;        // Written without decimals                        
var y = 123e5;      // 12300000
var z = 123e-5;     // 0.00123

Strings (or text strings) are a series of characters like "John Doe". Strings are written with quotes. You can use single or double quotes:

var carName = "Volvo XC60";              // Using double quotes
var carName = 'Volvo XC60';              // Using single quotes
var answer = "It's alright";             // Single quote inside double quotes
var answer = "He is called 'Johnny'";    // Single quotes inside double quotes
var answer = 'He is called "Johnny"';    // Double quotes inside single quotes

Strings include the empty string, "".

Booleans can have only two values, true or false:

var x = true;
var y = false;

true essentially is equal to 1
false essentially is equal to 0
The following is important in that true essentially means !0:

0 == false      // Is true
1 == true       // Is true
!0              // Is true
0               // Is false
1               // Is true
-1              // Is true (any number other than 0 is true)

Objects include ordinary objects, arrays and functions.

JavaScript objects are written with curly braces. Object properties are written as name:value pairs, separated by commas.

var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};

JavaScript Arrays are a type of object.

Arrays are written with square brackets.
Array items are separated by commas.
The following code declares (creates) an array called cars, containing three items (car names):

var cars = ["Saab", "Volvo", "BMW"];

Array indexes are zero-based, which means the first item is [0], second is [1], and so on.

Typeof - Can use the JavaScript typeof operator to determine the type of a variable:

typeof "John"                // Returns "string" 
typeof 3.14                  // Returns "number"
typeof false                 // Returns "boolean"
typeof [1,2,3,4]             // Returns "object" (not "array", see note below)
typeof {name:'John', age:34} // Returns "object"

Undefined - In JavaScript, a variable without a value, has the value undefined. The typeof is also undefined.

var person;                  // Value is undefined, type is undefined

Any variable can be emptied, by setting the value to undefined. The type will also be undefined.

Null - In JavaScript null is "nothing". It is supposed to be something that doesn't exist.

Unfortunately, in JavaScript, the data type of null is an object.
You can consider it a bug in JavaScript that typeof null is an object. It should be null.
You can empty an object by setting it to null.

var person = null;           // Value is null, but type is still an object

You can also empty an object by setting it to undefined:

var person = undefined;     // Value is undefined, type is undefined

Differences between Null and Undefined:

typeof undefined             // undefined
typeof null                  // object
null === undefined           // false
null == undefined            // true

* Examples and content from W3Schools.com.