JavaScript isset() Equivalent

In JavaScript, there is no built-in “isset()” function like in some other languages, such as PHP. The isset() is the function in PHP that verifies whether a variable is set/declared and is not Null. It gives “true” if the variable is declared. Otherwise, its outputs “false”. In PHP, if the programmer does not use it, they face a “variable is not defined” error.

This article will describe the equivalent of the isset() function of PHP in JavaScript.

JavaScript isset() Equivalent

Although there is not an isset() equivalent in JavaScript, there are numerous ways to check if a variable is possibly undeclared. Some of those approaches are given below:

Method 1: Using the “typeof” Operator

In JavaScript, the “typeof” operator is equivalent to “isset()”. It accepts an operand/variable and gives the type of its value. If the variable is not declared, it outputs the “undefined”.


Use the following syntax for the typeof operator:

typeof variable


Declare a variable “x” and assign it a value:

var x = 11;

Call the typeof operator with the variable that tells the type of the value stored in a specified variable:

typeof x;


Let’s see what would be done if the variable is not declared:

typeof y;

The output shows “undefined” because the variable “y” is not declared:

If the value is not declared or declared but is not assigned, then, use the below-given lines of code to verify it:

if (typeof x !== 'undefined' && x !== null) {

console.log("The x is the " + typeof x + " (" + x + ")");



console.log("x is not defined");


The output shows the value and its type stored in the variable “x” because it is defined and not null:

Method 2: Using “hasOwnProperty()” Method

The “hasOwnProperty()” is equivalent to the isset() in JavaScript. It is used to determine whether a property is present inside an object. It returns “true” if the particular property is a direct property of the object or even though the value of the specified property is undefined.


Follow the given syntax to verify the object’s property defined in the object:



Create an object with its properties in a key-value pair:

var info = {

  name: 'Jenny',

  age: 24


Check if “rollno” is the property of the object “info” or not:


It can be seen that the hasOwnProperty returns “false” which means the specified property does not exist in the object:

Method 3: Using the “in” Operator

In JavaScript, you can use the “in” operator as an equivalent to the isset() in PHP. It determines if a property is present inside an object or the prototype chain of that object.


The following syntax is utilized for using the “in” operator:

property in Object


Check the property “age” in object “info” using the “in” operator:

console.log('age' in info);


That was all about the equivalent of isset() in JavaScript.


There is no built-in “isset()” function in JavaScript, but there are numerous ways to check if a variable is possibly undeclared, such as the “typeof” operator, “hasOwnProperty()” method, and the “in” operator. This blog discussed the equivalent of isset() in JavaScript with the help of practical examples.

About the author

Farah Batool

I completed my master's degree in computer science. I am an academic researcher and love to learn and write about new technologies. I am passionate about writing and sharing my experience with the world.