adding a custom type, typeMap is frozen about typechecker HOT 8 CLOSED

// custom-typechecker.js
const typechecker = require('typechecker')
const customTypeChecker = Object.assign({}, typechecker)

const customTypeMap = {
	array: typechecker.isArray,
	boolean: typechecker.isBoolean,
	date: typechecker.isDate,
	error: typechecker.isError,
	class: typechecker.isClass,
	function: typechecker.isFunction,
	null: typechecker.isNull,
	number: typechecker.isNumber,
	regexp: typechecker.isRegExp,
	string: typechecker.isString,
	'undefined': typechecker.isUndefined,
	buffer: Buffer.isBuffer,  // must be before isObject, as otherwise isObject will catch it
	map: typechecker.isMap,
	weakmap: typechecker.isWeakMap,
	object: typechecker.isObject
}

customTypeChecker.getType = function customGetType (value, _typeMap) {
	return typechecker.getType(value, _typeMap || customTypeMap)
}

module.exports = customTypeChecker

// app.js
var result = require('./custom-typechecker.js').getType(Buffer.from(''))
console.log(result)  // buffer

// Add our custom types
const customTypeMap = {
	buffer: Buffer.isBuffer
}
// Add original types
Object.keys(typechecker.typeMap).forEach(function (key) {
	customTypeMap[key] = customTypeMap[key] || typechecker.typeMap[key]
})

//mytypechecker.js

const typechecker = require('typechecker')
const customTypeChecker = Object.assign({}, typechecker)

// add custom types, a key and corresponding function for each
customTypeChecker.isBuffer = function isBuffer(obj) {
  return !!obj.constructor && typeof obj.constructor.isBuffer === 'function' && obj.constructor.isBuffer(obj)
}

// Create a custom type map and make an key/property for each new type 
// the key/property used will be the string returned by getType. 
let customTypeMap = {
  buffer: customTypeChecker.isBuffer
}

// merge base typeMap with customTypeMap
customTypeMap = Object.assign(customTypeMap, customTypeChecker.typeMap)

//overwrite getType to use customTypeMap if available
customTypeChecker.getType = function customGetType(value, _typeMap) {
  return typechecker.getType(value, _typeMap || customTypeMap)
}

module.exports = customTypeChecker