LSP — The Liskov Substitution Principle

The Liskov Substitution Principle is a part of SOLID, a mnemonic acronym that bundles a total of 5 design principles.

It is often associated with clean code.

But what exactly is it, is it important to you, and should you even care?

What does it state?

If S is a subtype of T, then objects of type T may be replaced with objects of type S without altering any of the desirable properties of the program (correctness, task performed, etc.).

Sounds complicated, right?

Well, it can be boiled down to a simpler definition:

Software (systems) should be built from interchangeable parts. Those parts should agree on a common contract, which enables those parts to be substituted one for another.

Still maybe not that easy, but let’s go one step further, and take a look at it from the perspective of JavaScript:

✅ Methods of a subclass that override methods of a base class must have exactly the same number of arguments

✅ Each argument of the overriding method must have the same type as in the method of the base class

✅ The return type of a method overriding a base method must be of the same type