How do we get people to understand programming? Khan Academy recently launched an online environment for learning to program.
Often we need to represent an idea or concept in our programs — maybe a car engine, a computer file, a router, or a temperature reading. Representing these concepts directly in code comes in two parts: ES6 classes give us a convenient syntax for defining the state and behavior of objects that will represent our concepts.
ES6 classes make our code safer by guaranteeing that an initialization function will be called, and they make it easier to define a fixed set of functions that operate on that data and maintain valid state. Consider this non-class code. How many errors can you find? How would you fix them?
Notice also that, when adding a day, we checked in one place if we went beyond 31 but missed that check in another place. It would be better if we interacted with the data only through a small and fixed set of functions that each maintain valid state.
When a function is associated with a class or object, we call it a method. When an object is created from a class, that object is said to be an instance of the class.
Constructors The constructor method is special, and it solves the first problem. Keep Data Private We try to design our classes so that their state is guaranteed to be valid.
We provide a constructor that creates only valid values, and we design methods that also always leave behind only valid values. But as long as we leave the data of our classes accessible to everyone, someone will mess it up.
We protect against this by keeping the data inaccessible except through the functions we supply. Keeping data private to protect it is called encapsulation.
We have to fake them. The most common way to do that is to adhere to a simple convention: We used this approach in the earlier code example. Generally this simple convention works, but the data is technically still accessible to everyone, so we have to rely on our own discipline to do the right thing.
Privacy with Privileged Methods The next most common way to fake private object properties is to use ordinary variables in the constructor, and capture them in closures.
When a function is called as a method using the object. We call these static properties and static methods, respectively. There will only be one copy of a static property rather than a new copy per instance: Inheritance can avoid duplication and simplify the implementation of a class that needs the same data and functions as another class.
Inheritance also allows us to substitute subclasses, relying only on the interface provided by a common superclass. Inherit to Avoid Duplication Consider this non-inheritance code: We could eliminate that repetition by having our Manager class inherit from the Employee class.
When we do, the state and behavior of the Employee class — its data and functions — will be incorporated into our Manager class. Notice the use of super:JARGON TIP: When a function is associated with a class or object, we call it a method.; When an object is created from a class, that object is said to be an instance of the class.
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