Tommy Ku's Method Stub

Reading and thinking

The "cat state" of boolean variable

Posted on 2017.11.15

One day when I was implementing a login UI of my app in React, I thought about a let login = true flag.

For a boolean variable in an object-oriented language such as JavaScript, how many states could it possibly have? Let’s count, true and false, there gives you two states.

S ∈ { True, False }

Nope, there is an extra state, null, the variable can point to an empty reference. Of course, in such case we aren’t talking about the variable itself, we are talking about a reference name. In JavaScript, such reference could also evaluate to undefined!

S ∈ { True, False, null, undefined }

Why does it matter?

Consider the login flag we mentioned, the following table summarizes all possible values and component it should render as a result.

login Component
true IndexPage
false LoginPage
null SplashPage
undefined SplashPage

Rendering IndexPage and LoginPage when login is true or false looks straightforward, but null? How about undefined? One trick I took is to make use of the non-determinism of the variable, to wit:

I don’t know whether it’s true or false, so it’s in a “cat state” that’s neither true nor false.

In other words,

S ∈ { True, False, Cat }

Should I use it?

Don’t do that. There exists better alternative without additional complexity, and by using the cat state you make your code harder to understand.

Alternatives such as enum, like a set of constants encapsulate exactly what this non-deterministic boolean variable is trying to do. For example:

// LoginState.js
export DONNO = 1;
export NOT_LOGGEDIN = 1;
export LOGGEDIN = 2;

// app.js
import LoginState from 'LoginState'

let login = LoginState.DONNO;
let renderComponent;

switch (login) {
  case LoginState.NOT_LOGGEDIN:
    renderComponent = LoginPage;
  case LoginState.LOGGEDIN:
    renderComponent = IndexPage;
    renderComponent = SplashPage;

A quick-and-dirty implementation using non-determinism of boolean variable wouldn’t save you much more lines of code than that.

As my professor used to say:

[insert programming language] gives you lots of ropes to hang yourself.

In this case, the rope is to make use of a boolean variable’s cat state and relying on it to decide the next action. You can do that of course, but I have shown above there is a better way to do that.

When we evaluate a boolean variable, string like '123', or integer 3 could both be evaluated to true, while null, and undefined both are evaluated to false. However, you shouldn’t rely on it because it’s inconsistent.

Consider the following if-statement:

if (login === true) {
  // ...logged in
} else {
  // state 🐱

Our understanding on boolean variable is that it’s value could either be true or false, there is no cat state before OOP, and now with cat state we are just inviting troubles because to interpret the if-statement above, we must also consider what that boolean could be — an integer, a string, a null, or an undefined?

If the codebase has never relied on null or undefined in the first place for it’s logic, then we could even forgo the === true part (note 1 == true evaluates to true).


Using the “cat state” of a boolean variable might be a neat trick on project where everyone working on the code is aware of the possible values of that variable might contain. It could be true, false, and anything else.

A safer bet, would be to use boolean strictly with its true/false values by using === operator, or use TypeScript that comes with static type-checking.