Logical operators — JavaScript fundamentals

• AND &&
• OR ||
• Negation

You can combine logical expressions to create increasingly cleverer and more useful checks. One good example is password verification. As you know, some websites want a password of 8 to 20 characters on signup. Frankly, it's a weird restriction, but whatever, it is what it is. In math, we would write 8 < x < 20 (where x is the length of a particular password), but that trick won't work in JavaScript. We would have to make two separate logical expressions and connect them with the special "AND" operator:

A password longer than 8 characters **AND** a password shorter than 20 characters.

Here's a function that takes the password and says whether it meets the conditions or not:

const isStrongPassword = (password) => {
  const length = password.length;
  return length > 8 && length < 20;
};

isStrongPassword('qwerty'); // false
isStrongPassword('qwerty1234'); // true
isStrongPassword('zxcvbnmasdfghjkqwertyui'); // false

&& means "AND" (called a conjunction in mathematical logic). The whole expression is true only when every operand, which are all part of the compound expressions, is true. In other words, && means "both".

This operator's priority is lower than that of comparison operators, so the expression works correctly without parentheses.

Another widespread operator along with && is || — "OR" (disjunction). It means "one or the other, or both". Operators can be combined in any number and any sequence, but when && and || appear together, you should label priority with parentheses. Below is an example of an advanced function validating a password:

const hasSpecialChars = (str) => /* checks for special characters in the string */;

const isStrongPassword = (password) => {
  const length = password.length;
  // The parentheses set the priority, making it clear how each part is related
  return (length > 8 && length < 20) || hasSpecialChars(password);
};

Another example. We want to buy an apartment that meets these conditions: an area of 100 square meters or more on any street OR an area of 80 square meters or more, but on Main Street.

We'll write a function checking the apartment. It takes two arguments: the area (a number) and the street name (a string):

const isGoodApartment = (area, street) => {
  // Via a variable to make sure the function is not too long
  const result = area >= 100 || (area >= 80 && street === 'Main Street');
  return result;
};

isGoodApartment(91, 'Queens Street'); // false
isGoodApartment(78, 'Queens Street'); // false
isGoodApartment(70, 'Main Street');   // false

isGoodApartment(120, 'Queens Street'); // true
isGoodApartment(120, 'Main Street');   // true
isGoodApartment(80, 'Main Street');    // true

The area of mathematics dealing with logical operators is called Boolean algebra. The "truth tables" are shown below and can be used to figure out the result of an operator:

AND &&

Few examples:

// true && true;
3 > 2 && 'wow'.startsWith('w'); // true
// true && false;
'start' === 'start' && 8 < 3; // false

OR ||

Few examples:

// true || true;
3 > 2 || 'wow'.startsWith('w'); // true
// false || false;
'start' === 'Start' || 3 < 3; // false

Negation

Along with the conjunction (AND) and disjunction (OR) we often use the "negation" operator, too. It changes the logical value to the opposite one. It is denoted by the unary operator ! in programming.

If there is a function that checks for even numbers, then negation will allow you to check for odd numbers:

const isEven = (number) => number % 2 === 0;
isEven(10);  // true
!isEven(10); // false

I.e., we can just put a ! to the left of the function call and get the opposite result.

Negation is a powerful tool that allows you to concisely express the desired rules in your code without having to write new functions.

What if you wrote !!isEven(10) like this? Suddenly, the code would work. Double negation in logic is equivalent to no negation at all.

isEven(10);   // true
!isEven(10);  // false
!!isEven(10); // true

Recommended materials

1. Boolean algebra
2. Conjunction
3. Disjunction
4. De Morgan's laws