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:

• Using the typeof operator
• Using hasOwnProperty() method
• Using the in operator

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”.

Syntax

Use the following syntax for the typeof operator:

Example

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

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

Output

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

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:

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.

Syntax

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

Example

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

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.

Syntax

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

Example

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

Output

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

Conclusion

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.