# Inheritance

> Source: https://learn-typescript.org/inheritance/
> Part of Learn TypeScript, free to read.

JavaScript uses prototype based inheritance. Every object has a `prototype`, and when a method of the object is called then JavaScript tries to find the right function to execute from the prototype object.

### The prototype attribute

Without using the prototype object, we can define the object Person like this:

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

    function describe()
    {
        return this.name + ", " + this.age + " years old.";
    }
}
```

When creating instances of the `Person` object, we create a new copy of all members and methods of the functions. This means that every instance of an object will have its own `name` and `age` properties, as well as its own `describe` function.

However, if we use the `Person.prototype` object and assign a function to it, it will also work.

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

Person.prototype.describe = function()
{
    return this.name + ", " + this.age + " years old.";
}
```

When creating instances of the `Person` object, they will not contain a copy of the `describe` function. Instead, when calling an object method, JavaScript will attempt to resolve the `describe` function first from the object itself, and then using its `prototype` attribute.

### Inheritance

Let’s say we want to create a `Person` object, and a `Student` object derived from `Person`:

```typescript
let Person = function() {};
 
Person.prototype.initialize = function(name, age)
{
    this.name = name;
    this.age = age;
}

Person.prototype.describe = function()
{
    return this.name + ", " + this.age + " years old.";
}

let Student = function() {};
Student.prototype = new Person();

Student.prototype.learn = function(subject)
{
    console.log(this.name + " just learned " + subject);
}

let me = new Student();

me.initialize("John", 25);
me.learn("Inheritance");
```

As we can see in this example, the `initialize` method belongs to `Person` and the `learn` method belongs to `Student`, both of which are now part of the `me` object.

Keep in mind that there are many ways of doing inheritance in JavaScript, and this is just one of them.