A JavaScript constructor function is a function that is used to create new objects. It is called a constructor because it constructs new objects.

To create a JavaScript constructor function, you use the function keyword. The function name should be descriptive of the type of object that the function will create. For example, a constructor function for a Person object might be called Person.

The body of the constructor function should contain the code that is executed when a new object is created. This code can be used to initialize the properties of the new object.

Here is an example of a JavaScript constructor function that creates a Person object:

<script type="text/javascript">
        function Person(person_name, person_age, person_gender) {
            // assigning  parameter values to the calling object
            this.name = person_name,
                this.age = person_age,
                this.gender = person_gender,
                this.greet = function() {
                    return ('Hi' + ' ' + this.name);
                }
        }
        // creating objects
        const person1 = new Person('Amit', 23, 'male');
        const person2 = new Person('Sumit', 25, 'female');
        // accessing properties
        console.log(person1.name); 
        console.log(person2.name); 
    </script>

Output:-