/**
 * An extended version of the `Event` emitted by the `EventSource` object when an error occurs.
 * While the spec does not include any additional properties, we intentionally go beyond the spec
 * and provide some (minimal) additional information to aid in debugging.
 *
 * @public
 */
export declare class ErrorEvent extends Event {
    /**
     * HTTP status code, if this was triggered by an HTTP error
     * Note: this is not part of the spec, but is included for better error handling.
     *
     * @public
     */
    code?: number | undefined;
    /**
     * Optional message attached to the error.
     * Note: this is not part of the spec, but is included for better error handling.
     *
     * @public
     */
    message?: string | undefined;
    /**
     * Constructs a new `ErrorEvent` instance. This is typically not called directly,
     * but rather emitted by the `EventSource` object when an error occurs.
     *
     * @param type - The type of the event (should be "error")
     * @param errorEventInitDict - Optional properties to include in the error event
     */
    constructor(type: string, errorEventInitDict?: {
        message?: string | undefined;
        code?: number | undefined;
    });
}
/**
 * For environments where DOMException may not exist, we will use a SyntaxError instead.
 * While this isn't strictly according to spec, it is very close.
 *
 * @param message - The message to include in the error
 * @returns A `DOMException` or `SyntaxError` instance
 * @internal
 */
export declare function syntaxError(message: string): SyntaxError;
/**
 * Flatten an error into a single error message string.
 * Unwraps nested errors and joins them with a comma.
 *
 * @param err - The error to flatten
 * @returns A string representation of the error
 * @internal
 */
export declare function flattenError(err: unknown): string;
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