Prototype inheritance not working: Cannot create property 'firstName' on string

There are a couple of issues here:

person2.prototype = new person()

is wrong way of inheritance ( More on that here )

It should be

person2.prototype = Object.create(person.prototype);
person2.prototype.constructor = person2;

person.prototype.getfullName should be declared outside of person constructor function. Otherwise, every instance created using new will update the method on the prototype

(Reference: here, here and here)

function person(firstName, lastName) {
  this.firstName = firstName;
  this.lastName = lastName;
}

person.prototype.getfullName = function() {
  return `${this.firstName} ${this.lastName}`
}

The first argument to the .call is the this value to be used inside the person function. So, it should be

person.call(this, firstName, lastName)

Here's the updated snippet:

function person(firstName, lastName) {
  this.firstName = firstName;
  this.lastName = lastName;
}

person.prototype.getfullName = function() {
  return `${this.firstName} ${this.lastName}`
}

function person2(firstName, lastName, age, location) {
  person.call(this, firstName, lastName)
  this.age = age;
  this.location = location;
}

person2.prototype = Object.create(person.prototype);
person2.prototype.constructor = person2;

let result = new person2('nivedhan', 'kumar', 24, 'neyveli')

console.log(result.getfullName())
console.log(result.age)

call allows us to manually and explicitly set the this keyword of any function we want to call. call() provides a new value of this to the function/method. With call() we can write a method once and then inherit it in another object, without having to rewrite the method for the new object.

const user = {
  name: 'john',
  setName: function(name) {
    this.name = name;
  },
  getName: function() {
    return this.name;
  }
}
const user2 = {
  name: 'sam'
}
const getName = user.getName;
const name = getName.call(user2); //return john

console.log(name)