isBoolean

Determines whether a value is a boolean type with enhanced type safety. Provides reliable boolean type detection using native typeof check, distinguishing true boolean values from boolean-like values or truthy/falsy expressions.

Signature

const isBoolean: (value?: unknown) => value is boolean

Parameters

Name	Type	Description
value	-	Value to test for boolean type

Returns

Type-safe boolean indicating whether the value is a boolean

Examples

Boolean type detection

import { isBoolean } from '@winglet/common-utils';

// True cases - actual boolean values
console.log(isBoolean(true)); // true
console.log(isBoolean(false)); // true
console.log(isBoolean(Boolean(1))); // true
console.log(isBoolean(!!42)); // true

// False cases - not boolean type
console.log(isBoolean(1)); // false (truthy but not boolean)
console.log(isBoolean(0)); // false (falsy but not boolean)
console.log(isBoolean('true')); // false (string)
console.log(isBoolean('false')); // false (string)
console.log(isBoolean(null)); // false
console.log(isBoolean(undefined)); // false
console.log(isBoolean([])); // false (truthy but not boolean)
console.log(isBoolean({})); // false (truthy but not boolean)

Configuration validation

interface AppConfig {
  debug?: unknown;
  enableFeature?: unknown;
  autoSave?: unknown;
}

function validateBooleanConfig(config: AppConfig) {
  const validConfig: Record<string, boolean> = {};

  for (const [key, value] of Object.entries(config)) {
    if (isBoolean(value)) {
      validConfig[key] = value;
    } else {
      console.warn(`Config ${key} should be boolean, got:`, typeof value);
      // Apply default or convert
      validConfig[key] = Boolean(value);
    }
  }

  return validConfig;
}

Form data processing

function processBooleanField(formValue: unknown): boolean {
  if (isBoolean(formValue)) {
    return formValue;
  }

  // Handle string representations
  if (typeof formValue === 'string') {
    const lower = formValue.toLowerCase();
    return lower === 'true' || lower === '1' || lower === 'yes';
  }

  // Convert other types
  return Boolean(formValue);
}

// Usage
console.log(processBooleanField(true)); // true
console.log(processBooleanField('true')); // true
console.log(processBooleanField('false')); // false
console.log(processBooleanField(1)); // true

API response validation

interface ApiFlags {
  isEnabled: unknown;
  hasAccess: unknown;
  isPublic: unknown;
}

function validateApiFlags(flags: ApiFlags) {
  const errors: string[] = [];

  if (!isBoolean(flags.isEnabled)) {
    errors.push('isEnabled must be boolean');
  }

  if (!isBoolean(flags.hasAccess)) {
    errors.push('hasAccess must be boolean');
  }

  if (!isBoolean(flags.isPublic)) {
    errors.push('isPublic must be boolean');
  }

  if (errors.length > 0) {
    throw new Error(`Validation failed: ${errors.join(', ')}`);
  }

  return flags as { isEnabled: boolean; hasAccess: boolean; isPublic: boolean };
}

Type guard usage

function processValue(value: unknown) {
  if (isBoolean(value)) {
    // TypeScript knows value is boolean
    console.log(`Boolean value: ${value ? 'true' : 'false'}`);
    return value ? 1 : 0;
  }

  console.log('Not a boolean, converting...');
  return Boolean(value) ? 1 : 0;
}

Playground

import { isBoolean } from '@winglet/common-utils';

// True cases - actual boolean values
console.log(isBoolean(true)); // true
console.log(isBoolean(false)); // true
console.log(isBoolean(Boolean(1))); // true
console.log(isBoolean(!!42)); // true

// False cases - not boolean type
console.log(isBoolean(1)); // false (truthy but not boolean)
console.log(isBoolean(0)); // false (falsy but not boolean)
console.log(isBoolean('true')); // false (string)
console.log(isBoolean('false')); // false (string)
console.log(isBoolean(null)); // false
console.log(isBoolean(undefined)); // false
console.log(isBoolean([])); // false (truthy but not boolean)
console.log(isBoolean({})); // false (truthy but not boolean)

Notes

Type Safety:

• Distinguishes actual boolean type from truthy/falsy values
• Provides TypeScript type narrowing
• More precise than !!value or Boolean(value) for type checking

Use Cases:

• Configuration validation
• API response validation
• Form data type checking
• Type guards before boolean operations
• Strict type validation in utilities

Performance: Direct typeof check provides optimal performance.

Related Functions:

• Use isTruthy() for truthy value detection
• Use Boolean() for type conversion
• Use !!value for quick boolean conversion