How to return the data type of variable in JavaScript ?

To return the JavaScript data type of a variable we can use the JavaScript typeof operator. In JavaScript, unlike many other programming languages, we do not specify the type of a variable while declaring it, rather the variable’s type is automatically inferred based on the value it holds. In other words, JavaScript is a “dynamically typed” programming language. In such languages, the type of a variable can change throughout the program.

Example: In this example, x was initialized to a number, then we initialized it to a string, and then an object. This makes it difficult to keep track of the type of variable ‘x’ throughout the program.

Javascript

// x is a number
let x = 4242;
console.log(x);

// x is a string
x = "GeeksforGeeks";
console.log(x)

// x is an object
x = {
    k: 4245,
    a: "geeks"
};
console.log(x)

Output

4242
GeeksforGeeks
{ k: 4245, a: 'geeks' }

JavaScript typeof Operator

The typeof keyword helps to determine the type of a variable in Javascript. Since Javascript is a dynamically typed programming language, typeof can be used to find the variable type.

It can be used within a function to check the data type of a variable or to check if a variable is declared. Let’s consider the following examples to understand this better.

Example 1: In this example, we will print the type of x in the console.

Javascript

let x = 12345;
console.log(typeof(x));

Output

number

Example 2: In this example, we will print the type of a string in the console.

Javascript

let x = "GeeksforGeeks";
console.log(typeof(x));

Output

string

Output:

string

Example 3: In this example, we will print the type of various variables in the console.

Javascript

let x = { k: 12, m: "geeky stuff" }
console.log(typeof (x))
console.log(typeof (x.k))
console.log(typeof (x.m))
console.log(typeof (x.s))

Output

object
number
string
undefined

A common use of typeof operator is to determine the variable type and perform actions accordingly within a function.

Example: Invoke the above function with a number and string as an argument. Another use of the typeof operator is to check if a variable is declared before its usage.

Javascript

function doX(x) {
    if (typeof (x) === "number") {
        console.log("x is a number")
    }
    if (typeof (x) === "string") {
        console.log("x is a string")
    }
    if (typeof (x) === "undefined") {
        console.log("x is undefined")
    }
}

doX(12)
doX("hello gfg")

Output

x is a number
x is a string

Example: Invoking the above function without passing an argument and by passing a string as an argument.

Javascript

function checkX(x) {
    if (typeof (x) === "undefined") {
        console.log(
            "x is undefined. Please declare it");
    } else {
        console.log("We can process x!")
    }
}

checkX()
checkX("hello")

Output

x is undefined. Please declare it
We can process x!

Example: One small caveat with typeof is that typeof(NaN) returns a number. When we multiply a string with a number we get NaN, as seen in the below example.

Javascript

let x = "hello"
console.log(x)

let y = 10
console.log(y)

z = x * y
console.log(z)
console.log(typeof (z))

Output

hello
10
NaN
number