How to Check Type of Data in Javascript

Javascript provides 2 operators to check the type of a given value :

• typeof : This checks whether the value is one of the primitive data types. It will return a string specifying the type — "undefined" / "string" / "number" / "boolean" / "object" etc.

• instanceof : This checks the "kind" of an object. For example, Javascript arrays are basically objects. Using this we can check whether a given value represents an array.
  instanceof will return a boolean true / false depending on whether the value is an instance of a given object or not.

Checking for undefined

typeof operator will return "undefined" for an undefined value.

var a;

// "Undefined value"
if(typeof a == "undefined")
	console.log("Undefined value");
else
	console.log("Defined value");

Alternatively you can use the strict equality operator to check for undefined.

var a;

// "Undefined value"
if(a === undefined)
	console.log("Undefined value");
else
	console.log("Defined value");

Checking for null

Unfortunately due to a Javascript bug, typeof operator on null will return the type as "object".

So we need to use the strict equality operator === to check for a null.

var b = null;

// "Null value"
if(b === null)
	console.log("Null value");
else
	console.log("Not null value");

Checking for a Number

typeof operator will return "number" for a numeric value.

var a = 5;

// "Numeric value"
if(typeof a == "number")
	console.log("Numeric value");
else
	console.log("Not a number");

Checking for a String

typeof operator will return "string" for a string of characters.

var a = 'Tree';

// "String value"
if(typeof a == "string")
	console.log("String value");
else
	console.log("Not a string");

Checking for a Boolean

typeof operator will return "boolean" for boolean values (true / false).

var a = true;

// "Boolean value"
if(typeof a == "boolean")
	console.log("Boolean value");
else
	console.log("Not a boolean");

Checking for an Object

typeof operator will return "object" if the value represents an object.

var a = { key: "value" };

// "Object"
if(typeof a == "object")
	console.log("Object");
else
	console.log("Not an object");

Checking for an Array

typeof operator on an array will return "object" (as Javascript array is an object internally). However sometimes we need to check whether the value represents an array or not.

Ths can be done with the instanceof operator. This will check whether the value is a specific "kind" of an object.

var a = [1, 2, 3];

// "Represents an array"
if(a instanceof Array)
	console.log("Represents an array");
else
	console.log("Does not represent an array");