Retriever
A retriever in LlamaIndex is what is used to fetch Nodes from an index using a query string. Aa VectorIndexRetriever will fetch the top-k most similar nodes. Meanwhile, a SummaryIndexRetriever will fetch all nodes no matter the query.
const retriever = vector_index.asRetriever();
retriever.similarityTopK = 3;
// Fetch nodes!
const nodesWithScore = await retriever.retrieve("query string");