firebase pagination

pagination.ts

//Typescript version
class Pagination {
    public dataRef: string;
    public perPage: number;
    public order: string;
    public hasRecords: boolean = true;

    private _pageRecordCount: number = 0;
    private _cursor: string = "";

    constructor(dataRef: string, perPage?: number, order?: string){
        this.dataRef = dataRef;
        this.perPage = perPage || 10;
        this.order = order || 'dsc';
    }

    private _handleData(data: any){
        let currentRecords = [];
        this.hasRecords = false;
        this._pageRecordCount = 0;
        this._cursor = "";
        if(data){
            let entities = data;
            let entityKeys = Object.keys(entities);

            if(entityKeys.length > this.perPage){
                (this.order == 'asc') ? (this._cursor = entityKeys.pop()) : (this._cursor = entityKeys.shift());
                this.hasRecords = true;
            }
            
            for(let entity of entityKeys){
                this._pageRecordCount++;
                if(this._pageRecordCount <= this.perPage){
                    let tempData = entities[entity];
                    tempData["id"] = entity;
                    (this.order == 'asc') ? (currentRecords.push(tempData)) : (currentRecords.unshift(tempData));
                }
            }
        }
        return new Promise((resolve, reject) => {
            resolve(currentRecords);
        });
    }

    public fetchRecords(){
        let databaseRef = firebase.database().ref(this.dataRef);
        let orderByRef = databaseRef.orderByKey();
        let filterRef;
        let cursorRef;

        if(this._cursor){
            cursorRef = (this.order == 'asc') ? orderByRef.startAt(this._cursor) : orderByRef.endAt(this._cursor);
        }else{
            cursorRef = orderByRef;
        }

        if(this.order == 'asc'){
            filterRef = cursorRef.limitToFirst(this.perPage + 1);
        }else {
            filterRef = cursorRef.limitToLast(this.perPage + 1);
        }

        return filterRef.once("value").then((data) => {
            return this._handleData(data.val());
        });
        
    }
}