This example goes over how to load data from JSONLines or JSONL files. The second argument is a JSONPointer to the property to extract from each JSON object in the file. One document will be created for each JSON object in the file. Example JSONLines file: 
{"html": "This is a sentence."}
{"html": "This is another sentence."}
Example code: 
import { JSONLinesLoader } from "langchain/document_loaders/fs/json";

const loader = new JSONLinesLoader(
  "src/document_loaders/example_data/example.jsonl",
  "/html"
);

const docs = await loader.load();
/*
[
  Document {
    "metadata": {
      "blobType": "application/jsonl+json",
      "line": 1,
      "source": "blob",
    },
    "pageContent": "This is a sentence.",
  },
  Document {
    "metadata": {
      "blobType": "application/jsonl+json",
      "line": 2,
      "source": "blob",
    },
    "pageContent": "This is another sentence.",
  },
]
*/