LSP — The Liskov Substitution Principle

3 min readJan 23, 2022

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

✅ It must only throw the same types of exceptions that the base class does

Some examples

To get an even better understanding of what exactly LSP is all about, you can take a look at the following examples, which illustrate violations of the principle.

Example 1

This example violates the LSP because the subclass FriendlyGreeter adds another parameter to the greet method.

class Greeter {
  greet(name) {
    return `Hello, ${name}!`;
  }
}class FriendlyGreeter extends Greeter {
  greet(name, age) {
    return `Hello, ${age} year old ${name}, have a nice day!`;
  }
}