In JavaScript, constructor functions are used to create and initialize objects. They are useful when you need to create multiple instances of an object with the same properties and methods.

To create a constructor function, you use the function keyword followed by the name of the function. Inside the function, you define the properties and methods of the object using the this keyword. Here’s an example:

function Person(name, age) {
  this.name = name;
  this.age = age;
}

Person.prototype.greet = function() {
  return "Hello, my name is " + this.name + " and I am " + this.age + " years old.";
}

var john = new Person("John", 25);
console.log(john.greet()); // Output: Hello, my name is John and I am 25 years old.

In the code snippet above, we define a Person constructor function that takes name and age as parameters. Inside the function, we assign these parameters to the name and age properties of the object using the this keyword.

We also add a greet method to the Person prototype, which returns a greeting string using the name and age properties.

To create an instance of the Person object, we use the new keyword followed by the constructor function name and pass the required arguments. In this case, we create a new Person object called john with the name “John” and age 25.

Finally, we can call the greet method on the john object to get the desired output.

By using constructor functions and prototypes, you can easily create and manipulate objects in JavaScript. It provides a way to define reusable blueprints for creating similar objects with shared properties and methods.

For more information, you can refer to the MDN web docs on constructor functions.