isBoolean

The isBoolean utility is a type guard that checks if a given value is a boolean primitive (true or false).

Implementation

View Source Code

/**
 * Checks if the value is a boolean.
 *
 * @example
 * ```ts
 * isBoolean(true); // true
 * isBoolean(false); // true
 * isBoolean(123); // false
 * isBoolean('hello world'); // false
 * isBoolean({}); // false
 * isBoolean([]); // false
 * isBoolean(new Date()); // false
 * isBoolean(null); // false
 * isBoolean(undefined); // false
 * isBoolean(NaN); // false
 * ```
 *
 * @param arg - The argument to be checked.
 *
 * @returns `true` if the value is a boolean, else `false`.
 */
export function isBoolean(arg: unknown): arg is boolean {
  return typeof arg === 'boolean';
}

Features

• Isomorphic: Works in both Browser and Node.js.
• Type Guard: Automatically narrows types to boolean within conditional blocks.
• Strict Check: Only returns true for actual boolean primitives.

API

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

Parameters

• value: The value to check.

Returns

• true if the value is a boolean; otherwise, false.

Examples

Basic Usage

import { isBoolean } from '@vielzeug/toolkit';

isBoolean(true); // true
isBoolean(false); // true
isBoolean(1); // false
isBoolean('true'); // false

Type Guarding

import { isBoolean } from '@vielzeug/toolkit';

function process(val: unknown) {
  if (isBoolean(val)) {
    // val is narrowed to boolean
    return val ? 'Yes' : 'No';
  }
}

Implementation Notes

• Returns true if typeof value === 'boolean'.
• Throws nothing; safe for any input type.

See Also

• isNumber: Check if a value is a number.
• isString: Check if a value is a string.
• is: Unified type-checking engine.