Javascript What is constructor function

Question

tell me when js function will be called as constructor function ?

 

if we write a simple js function then it will be called as constructor function ?

 

function myFunction(a, b) {
    return a * b;
}

var x = myFunction(4, 3);

 so tell me myFunction() will be consider as constructor function ? if no then describe the reason.

 

tell me why below js function is consider as constructor function ?

function person(first, last, age, eye) {
    this.firstName = first;
    this.lastName = last;
    this.age = age;
    this.eyeColor = eye;
}
var myFather = new person("John", "Doe", 50, "blue");
var myMother = new person("Sally", "Rally", 48, "green");

 when new keyword will be used to call js function then it will be consider as constructor function ?

 

please guide me. thanks