JavaScript Logical Operations

Boolean logic and logical operations might not come up that often in JavaScript development, but when they do, you'll be glad you know how to use them. Let's explore some of the most common logical operations in JavaScript and how to implement them in a functional style.

Logical AND

The logical and (&&) operator returns true if both operands are true, otherwise it returns false.

const and = (a, b) => a && b;

and(true, true); // true
and(true, false); // false
and(false, false); // false

Logical OR

The logical or (||) operator returns true if at least one of the operands is true, otherwise it returns false.

const or = (a, b) => a || b;

or(true, true); // true
or(true, false); // true
or(false, false); // false

Logical NOT

The logical not (!) operator returns true if the operand is false, otherwise it returns false.

const not = a => !a;

not(true); // false
not(false); // true

Logical XOR

The logical xor (^) operator returns true if exactly one of the operands is true, otherwise it returns false.

const xor = (a, b) => (( a || b ) && !( a && b ));

xor(true, true); // false
xor(true, false); // true
xor(false, true); // true
xor(false, false); // false

Logical NOR

The logical nor operator is a combination of the logical not and logical or operators. It returns true if none of the operands are true, otherwise it returns false.

const nor = (a, b) => !(a||b);

nor(true, true); // false
nor(true, false); // false
nor(false, false); // true

Logical NAND

The logical nand operator is a combination of the logical not and logical and operators. It returns true if at least one of the operands is false, otherwise it returns false.

const nand = (a, b) => !(a&&b);

nand(true, true); // false
nand(true, false); // true
nand(false, false); // true

Implementing logical operations with functions

The logical operations we've seen so far work with boolean values, but what if we want to use them with functions? For example, what if we want to check if both of two functions return true for a given set of arguments? We'll have to alter them in such a way that they work with functions. For example, here's the implementation of the logical and, or, and not operators for functions:

// Logical and for functions
const both = (f, g) => (...args) => f(...args) && g(...args);
// Logical or for functions
const either = (f, g) => (...args) => f(...args) || g(...args);
// Logical not for functions
const complement = fn => (...args) => !fn(...args);

In fact, we can generalize this to implement any binary logical operator for functions, using higher-order functions.

const logicalOperator =
  operatorFn =>
  (f, g) =>
  (...args) =>
    operatorFn(f(...args), g(...args));

const and = (a, b) => a && b;
const or = (a, b) => a || b;

const both = logicalOperator(and);
const either = logicalOperator(or);

const isEven = num => num % 2 === 0;
const isPositive = num => num > 0;
const isPositiveAndEven = both(isEven, isPositive);
const isPositiveOrEven = either(isEven, isPositive);

isPositiveAndEven(4); // true
isPositiveAndEven(-2); // false
isPositiveOrEven(4); // true
isPositiveOrEven(-2); // true