JavaScript Data Types
var length = 16; // Number
var lastName = "Johnson"; // String
var x = {firstName:"John",
lastName:"Doe"}; // Object
The Concept of Data Types
In programming, data types is an important concept.
To be able to operate on variables, it is important to know
something about the type.
Without data types, a computer cannot safely solve this:
var x = 16 + "Volvo";
Does it make any sense to add "Volvo" to sixteen?
Will it produce an error or will it produce a result?
JavaScript will treat the example above as:
var x = "16" + "Volvo";
When adding a number and a string, JavaScript will treat the
number as a string.
Example
var x = 16 + "Volvo";
»
Example
var x = "Volvo" + 16;
»
JavaScript evaluates expressions from left to right.
Different sequences can produce different results:
JavaScript:
var x = 16 + 4 + "Volvo";
Result:
20Volvo
»
JavaScript:
var x = "Volvo" + 16 + 4;
Result:
Volvo164
»
In the first example, JavaScript treats 16 and 4 as numbers,
until it reaches "Volvo".
In the second example, since the first operand is a string,
all operands are treated as strings.
JavaScript Types are Dynamic.
JavaScript has dynamic types. This means that the same
variable can be used to hold different data types:
Example
var x; // Now x is undefined
var x = 5; // Now x is a Number
var x = "John"; // Now x is a String
JavaScript Strings
A string (or a text string) is a series of characters like
"John Doe".
Strings are written with quotes. You can use single or double
quotes:
Example
var carName = "Volvo XC60"; // Using double quotes
var carName = 'Volvo XC60'; // Using single quotes
You can use quotes inside a string, as long as they don't
match the quotes surrounding the string:
Example
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
You will learn more about strings later in this tutorial.
Example
var x1 = 34.00; // Written with decimals
var x2 = 34; // Written without decimals
Extra large or extra small numbers can be written with
scientific (exponential) notation:
Example
var y = 123e5; // 12300000
var z = 123e-5; // 0.00123
JavaScript Booleans
Booleans can only have two values: true or false.
Example
var x = true;
var y = false;
Booleans are often used in conditional testing.
You will learn more about conditional testing later in this
tutorial.
JavaScript Arrays
JavaScript 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):
Example
var cars = ["Saab",
"Volvo", "BMW"];
»
Array indexes are zero-based, which means the first item is
[0], second is [1], and so on.
You will learn more about arrays later in this tutorial.
JavaScript Objects
JavaScript objects are written with curly braces.
Object properties are written as name:value pairs, separated
by commas.
Example
var person = {firstName:"John",
lastName:"Doe", age:50, eyeColor:"blue"};
»
The object (person) in the example above has 4 properties:
firstName, lastName, age, and eyeColor.
You will learn more about objects later in this tutorial.
The typeof Operator
You can use the JavaScript typeof operator to find the type
of a JavaScript variable.
The typeof operator returns the type of a variable or an
expression:
Example
typeof "" // Returns "string"
typeof "John" // Returns "string"
typeof "John Doe" // Returns "string"
»
Example
typeof 0 // Returns
"number"
typeof 314 // Returns "number"
typeof 3.14 // Returns "number"
typeof (3) // Returns "number"
typeof (3 + 4) // Returns "number"
»
Primitive Data
A primitive data value is a single simple data value with no
additional properties and methods.
The typeof operator can return one of these primitive types:
string
number
boolean
null
undefined
Example
typeof "John" // Returns "string"
typeof 3.14
// Returns
"number"
typeof true // Returns "boolean"
typeof false // Returns "boolean"
»
Complex Data
The typeof operator can return one of two complex types:
function
object
Example
typeof [1,2,3,4] // Returns "object" (not
"array", see note below)
typeof {name:'John', age:34} // Returns
"object"
typeof function myFunc(){} // Returns "function"
»
The typeof operator returns "object" for arrays
because in JavaScript arrays are objects.
Undefined
In JavaScript, a variable without a value, has the value
undefined. The typeof is also undefined.
Example
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.
Example
person = undefined; // Value is undefined, type is
undefined
»
Empty Values
An empty value has nothing to do with undefined.
An empty string variable has both a value and a type.
Example
var car = ""; // The value is "", the
typeof is "string"
»
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:
Example
var person = null; // Value is null, but type is still an
object
»
You can also empty an object by setting it to undefined:
Example
var person = undefined; // Value is undefined, type is undefined
»
Difference Between Undefined and Null
typeof undefined
// undefined
typeof null
// object
null === undefined
// false
null == undefined
// true
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