/**
 * Checks the type of the specified value.
 *
 * @example checkType(Number, 4)
 * @example checkType(String, 'hi')
 * @example checkType(String, 4) // throws
 * @example checkType({ // custom checker
 *  checkType (value, checkType) {
 *    checkType(Number, value)
 *  }
 * }, 4)
 */
function checkType (type, value) {
  if (type === String && (typeof value !== 'string' || !(value instanceof String))) {
    throw new Error('Expected string.')
  } else if (type === Number && (typeof value !== 'number' || !(value instanceof Number))) {
    throw new Error('Expected number')
  } else if (type === Boolean && (typeof value !== 'boolean' || !(value instanceof Boolean))) {
    throw new Error('Expected boolean.')
  } else if (type === Date && !(value instanceof Date)) {
    throw new Error('Expected Date.')
  } else if (type === RegExp && !(value instanceof RegExp)) {
    throw new Error('Expected RegExp.')
  } else if (typeof type.checkType === 'function') {
    type.checkType(value, checkType)
  } else if (!(value instanceof type)) {
    throw new Error('Expected ' + type.name + '.')
  }
}