export type PromiseCallback<T> = () => Promise<T>;
export type QueueCallback<T> = PromiseCallback<T> | (() => T);
/**
 * SerialQueue runs 1 promise at a time, very useful for reducing load and working around lock files
 */
export declare class SerialQueue {
    private name;
    private debug;
    private items;
    private isProcessing;
    constructor(name: string, debug?: boolean);
    private processQueue;
    /**
     * Add a new promise callback to the queue
     *
     * in case you provide a key it will be used to de-duplicate against existing items in the queue
     * if there is an existing item, the callback of that item will be used and this function will
     * return the result of that callback instead
     */
    add<T>(callback: QueueCallback<T>, key?: string): Promise<T>;
}