Datatypes in 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:

var carName1 = "Volvo XC60";   // Using double quotes
                                      var carName2 = 'Volvo XC60';   // Using single quotes
                                    

Numbers: JavaScript has only one type of numbers. Numbers can be written with, or without decimals.

var x1 = 34.00;     // Written with decimals
var x2 = 34;        // Written without decimals

Booleans: Booleans can only have two values: true or false.

                                          var x = 5;
var y = 5;
var z = 6;
(x == y)       // Returns true
(x == z)       // Returns false

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):

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

Objects: 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"};
                                        
                                        

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.

                                          typeof ""             // Returns "string"
                                          typeof "John"         // Returns "string"
                                          typeof "John Doe"     // Returns "string"
                                          typeof 0              // Returns "number"
typeof 314            // Returns "number"
typeof 3.14           // Returns "number"
typeof (3)            // Returns "number"
typeof (3 + 4)        // Returns "number"
                                        
                                        

Undefined: A variable without a value, has the value undefined. The type is also undefined. Any variable can be emptied, by setting the value to undefined. The type will also be undefined.

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

Empty Values: An empty value has nothing to do with undefined. An empty string has both a legal value and a type.

  var car;    // Value is undefined, type is 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:You can also empty an object by setting it to undefined.

  var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
person = null;    // Now value is null, but type is still an object
var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
person = undefined;   // Now both value and type is undefined

Difference Between Undefined and Null undefined and null are equal in value but different in type:

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

J avaScript variables can hold many data types: numbers, strings, booleabs, arrays, objects and more: To be able to operate on variables, it is important to know something about the type.