/** Returns `true` for `null`, `undefined`, an empty/whitespace string, or a non-finite number. */
export declare const isEmpty: (value: string | number | null | undefined) => boolean;
/** Returns `true` if `value` is one of the {@link IdentifierType} enum values. */
export declare const isIdentifierType: (value: unknown) => boolean;
/**
 * Returns `true` if `value` is a valid object identifier kind (`object_id` or
 * `cio_object_id`). Objects use a different id vocabulary than people, so this
 * is intentionally separate from {@link isIdentifierType}.
 */
export declare const isObjectIdType: (value: unknown) => value is "object_id" | "cio_object_id";
/**
 * Build a URL query string from a map of parameters.
 *
 * `null` and `undefined` values are omitted (so optional params disappear
 * rather than serializing as empty). Keys and values are URL-encoded. Returns
 * a leading-`?` string (e.g. `?a=1&b=2`) when at least one param is present,
 * or an empty string when none are.
 */
export declare const buildQueryString: (params: Record<string, string | number | boolean | null | undefined>) => string;
/**
 * Minimal response shape attached to a {@link CustomerIORequestError}. Replaces
 * the previous `http.IncomingMessage` so the public error type stays portable
 * across runtimes that don't expose Node's stream-based response object.
 */
export interface ResponseLike {
    statusCode: number;
    headers: Record<string, string>;
    ok: boolean;
}
/**
 * Thrown when the Customer.io API responds with a non-2xx status.
 *
 * The error message is derived from the API response body when possible. The
 * raw status code, response, and body are exposed for programmatic handling
 * (e.g. retry on 5xx, ignore on 404).
 *
 * @remarks
 * This is only thrown for HTTP responses with a non-2xx status. Transport-level
 * failures (DNS, connection reset, refused) and timeouts are surfaced as the
 * native `fetch` errors — `TypeError('fetch failed')` (with the underlying
 * cause on `.cause`) and `DOMException('TimeoutError')` respectively — not as a
 * `CustomerIORequestError`.
 *
 * @example
 * ```ts
 * try {
 *   await cio.identify('123', { email: 'a@example.com' });
 * } catch (err) {
 *   if (err instanceof CustomerIORequestError && err.statusCode === 404) {
 *     // customer not found, fall through
 *   } else {
 *     throw err;
 *   }
 * }
 * ```
 */
export declare class CustomerIORequestError extends Error {
    /** HTTP status code returned by the API. */
    statusCode: number;
    /** Portable response metadata ({@link ResponseLike}: status, lowercased headers, `ok`). */
    response: ResponseLike;
    /** The raw response body as a string. May be empty. */
    body: string;
    static composeMessage(json: Record<string, any> | null): string;
    constructor(json: Record<string, any> | null, statusCode: number, response: ResponseLike, body: string);
}
export declare function pickDefined<T extends Record<string, unknown>>(source: T, keys: ReadonlyArray<keyof T>): Partial<T>;
/**
 * Thrown synchronously by SDK methods when a required parameter is missing.
 *
 * The `message` is always `"<paramName> is required"`.
 */
export declare class MissingParamError extends Error {
    constructor(param: string);
}