Category: validation Since: 1.0.0 Tags: validation, type-guard, boolean, typescript

isBoolean

Checks whether the given value is a boolean.

Usage

import { isBoolean } from "@petr-ptacek/js-core";

// Type checking
console.log(isBoolean(true)); // true
console.log(isBoolean(false)); // true
console.log(isBoolean(1)); // false
console.log(isBoolean("true")); // false
console.log(isBoolean(null)); // false

// Type guard usage
const value: unknown = true;

if (isBoolean(value)) {
  // value is now typed as boolean
  const inverted = !value;
}

Why This Utility Exists

While typeof value === 'boolean' works, this utility provides TypeScript type guard functionality that properly narrows the type to boolean. It's useful for validation, filtering, and type-safe generic functions.

Signature

function isBoolean(value: unknown): value is boolean;

Parameters

• value (unknown): The value to test.

Return Type

Returns a boolean indicating whether the value is a boolean. When true, TypeScript narrows the type to boolean.

Design Notes

The implementation uses typeof value === "boolean" for runtime checking, which correctly identifies both true and false while excluding truthy and falsy values of other types.

When To Use

Use isBoolean when you need to:

• validate that a value is a boolean
• type-narrow unknown values
• filter for boolean values specifically
• separate booleans from truthy/falsy values

When Not To Use

Avoid when:

• you're checking truthiness (use !!value instead)
• you need to allow both booleans and truthy values
• you're using typeof for string comparison (use typeof directly if no narrowing needed)

Summary

isBoolean provides a type guard for boolean values, enabling safe type narrowing in TypeScript.

See also: isPrimitive (check for any primitive type).