code: function getCredential(accessKey, region, requestDate) {
    if (!isString(accessKey)) {
      throw new TypeError('accessKey should be of type "string"')
    }
    if (!isString(region)) {
      throw new TypeError('region should be of type "string"')
    }
    if (!isObject(requestDate)) {
      throw new TypeError('requestDate should be of type "object"')
    }
    return `${accessKey}/${getScope(region, requestDate)}`
  }
docs: generate a credential string