How to iterate over database in NodeJS? - example

/** * Simple database mock * Data from https://en.wikipedia.org/wiki/List_of_potentially_habitable_exoplanets */ const exoplanets = [ {"id": 1, "object":"Proxima Centauri b","star":"Proxima Centauri","starType":"M6Ve","distance":4.22}, {"id": 2, "object":"Gliese 667 Cc","star":"Gliese 667 C", "starType":"M3V","distance":23.62}, {"id": 5, "object":"Kepler-442b","star":"Kepler-442","starType":"K?V","distance":1291.6}, {"id": 7, "object":"Kepler-452b","star":"Kepler-452","starType":"G2V","distance":1402}, {"id": 9, "object":"Wolf 1061c","star":"Wolf 1061","starType":"M3V","distance":13.8}, {"id": 11, "object":"Kepler-1229b","star":"Kepler-1229","starType":"M?V","distance":769}, {"id": 12, "object":"Kapteyn b","star":"Kapteyn","starType":"sdM1","distance":13}, {"id": 14, "object":"Kepler-62f","star":"Kepler-62","starType":"K2V","distance":1200}, {"id": 15, "object":"Kepler-186f","star":"Kepler-186","starType":"M1V","distance":561}, {"id": 16, "object":"Luyten b","star":"Luyten's Star","starType":"M3.5V","distance":12.36}, {"id": 17, "object":"TRAPPIST-1d","star":"TRAPPIST-1","starType":"M8V","distance":39}, {"id": 18, "object":"TRAPPIST-1e","star":"TRAPPIST-1","starType":"M8V","distance":39}, {"id": 20, "object":"TRAPPIST-1f","star":"TRAPPIST-1","starType":"M8V","distance":39}, {"id": 21, "object":"TRAPPIST-1g","star":"TRAPPIST-1","starType":"M8V","distance":39}, {"id": 22, "object":"LHS 1140 b","star":"LHS 1140","starType":"M?V","distance":40}, {"id": 24, "object":"Kepler-1638b","star":"Kepler-1638","starType":"G4V","distance":2491.83}, {"id": 25, "object":"Teegarden c*","star":"Teegarden's Star","starType":"M7V","distance":12.58} ]; /** * This is our database mock */ class Database { constructor() { this.data = exoplanets; } log(idGreaterThan, limit) { let wheresql = idGreaterThan ? ` WHERE id > ${idGreaterThan}` : ''; let limitsql = limit < Infinity ? ` LIMIT ${limit}` : ''; console.info(`DATABASE: SELECT * FROM exoplanets${wheresql} ORDER BY id ASC${limitsql}`); } /** * Select planets with id grater than idGreaterThan * @param {object} query * @param {number} query.idGreaterThan boundrary value of id * @param {number} query.limit limit of rows * @returns {Promise<Array>} list of planets */ async select(query) { const { idGreaterThan, limit } = { idGreaterThan: 0, limit: Infinity, ...query }; this.log(idGreaterThan, limit); const reply = []; for(let item of this.data) { if(item.id > idGreaterThan) { reply.push(item); } if(reply.length === limit) { break; } } return reply; } } /** * Just wait 3s */ function wait() { return new Promise((resolve) => { setTimeout(resolve, 3000); }); }

/** * Iterate over database * @generator * @function readDocuments * @param {number} limit maximum number of documents * @yields {array} list of planets */ async function* readDocuments(limit) { const db = new Database(); let lastId = 0; // initialize with min value let done = false; // indicates end of iteration while(!done) { // select batch of documents and wait for database response // TODO: error handling const result = await db.select({ idGreaterThan: lastId, limit: limit }); // get id of last document lastId = result[result.length - 1].id; // end iteration if there are less documents than limit if(result.length < limit) { done = true; } // yield result yield result } };

(async function run() { // Iterate over dataset for await(let documents of readDocuments(4)) { // Do something with planets const planets = documents.map(planet => planet.object).join(', '); console.info(`APP: Got ${documents.length} items from database: ${planets}`); await wait(); } })();