Dynamic Data Types

JavaScript has dynamic data types. The same JavaScript variable can be treated as having different data types depending on the context it is used in.

          var travelType;           // travelType is undefined
var travelType = 5;       // travelType is a Number
var travelType = "Car";   // travelType is a String

Internal JavaScript data types:

String
Number
Boolean
null
undefined

Note:

String

A string in JavaScript is series of characters enclosed in quotation marks. A string must be delimited by quotation marks of the same type, either single quotation marks ' or double quotation marks ".

          var name = "John Smith";
var type = 'customer';

You can use quotes inside a string, as long as they don't match the quotes surrounding the string.

          var responseText = "It was paid to 'John Smith'";
var responseText = 'It was paid to "John Smith"';

You can put a quote inside a string using the \ character.

          var responseText = 'It\'s okay.';

You can access a character in a string by its zero-based position index.

          var name = "John Smith";
var character = name[3]; // character == 'n'

In JavaScript a string is an object. See String for properties and methods.

Number

JavaScript has only one type of number. Large numbers can be written in scientific (exponential) notation.

          var pi = 3.14;
var amount = 314e5;     // 31400000
var factor = 314e-5;    // 0.00314

JavaScript interprets numeric constants as octal if they are preceded by a zero, and as hexadecimal if they are preceded by a zero and x.

          var x=0377;    // This is 255 in decimal
var y=0xFF;    // This is 255 in decimal

Important:

When you assign a number to a variable, do not put quotes around the value. If you put quotes around a numeric value, the variable content will be treated as a string.

Never write a number with a leading zero, unless you want an octal conversion.

In JavaScript a number is an object. See Number for properties and methods.

Boolean

Booleans can only have two values: true or false.

          var sent = true;
var paid = false;

In JavaScript a Boolean is an object. See Boolean for properties and methods.

null

null is a special keyword denoting an empty value.

A variable can be emptied by setting it to null.

          travelType = null;

undefined

This is a special keyword denoting an undefined value.

Before a variable is assigned a value it is undefined.

          var travelType;    // variable is undefined