De Morgan’s Laws in Software Engineering

Simplify your conditionals

Photo by Artem Sapegin on Unsplash

What Are De Morgan’s Laws?

De Morgan’s laws state that specific Boolean statements can be written in different ways to the same effect. The precise definition can be seen here. In my own words, however, De Morgan’s laws are as follows:

1. A group of negated ands is the same as negated group of ors.

(!a && !b && !c) === !(a || b || c)

2. A group of negated ors is the same as a negated group of ands.

(!a || !b || !c) === !(a && b && c)

This is cool, but how can this be used in software?

Checking for Missing a Minimum Requirement

The first rule expresses that a group of negated ands is the same as a negated group of ors. This can be used any time you want to take action when at least one condition must be met.

In the below example, we will check that at least a user name, user id, or user email is provided. If not, we will throw an error.

Here we express it as (!a && !b && !c):

const userName = null;
const userId = null;
const userEmail = null;
if(!userName && !userId && !userEmail)) {
  throw new Error('At least one user identifier must be passed');
}

Here we express it as !(a || b || c):

const userName = null;
const userId = null;
const userAlias = null;
if(!(userName || userId || userAlias)) {
  throw new Error('At least one user identifier must be passed');
}

By using the second method, we can simply read this as “at least one of these is required.”

Checking for Requirements

The second rule expresses that a group of negated ors is the same as a negated group of ands. This can be used any time you want to take action when there are a series of conditions that must be met.

In the below example, we will check that a sequence of requirements are provided. If not, we will throw an error.

Here we express it as (!a || !b || !c):

const requirementA = true;
const requirementB =…