Working with JSONL tables in TypeScript

Updated May 9, 2026

JSONL (JSON Lines) file handling with automatic format detection and high-performance data operations.

Installation

npm install fairspec

Getting Started

The JSONL format is handled by the JSON plugin, which provides:

loadJsonTable - Load JSONL files into tables
saveJsonTable - Save tables to JSONL files
JsonPlugin - Plugin for framework integration

For example:

import { loadJsonTable } from "fairspec"

const table = await loadJsonTable({ data: "table.jsonl" })
// Newline-delimited JSON objects

Basic Usage

Loading JSONL Files

import { loadJsonTable } from "fairspec"

// Load from local file
const table = await loadJsonTable({ data: "data.jsonl" })

// Load with explicit format
const table = await loadJsonTable({
  data: "data.jsonl",
  format: { name: "jsonl" },
})

// Load multiple files (concatenated)
const table = await loadJsonTable({
  data: ["part1.jsonl", "part2.jsonl"],
})

Saving JSONL Files

import { saveJsonTable } from "fairspec"

// Save as JSONL
await saveJsonTable(table, {
  path: "output.jsonl",
  format: { name: "jsonl" },
})

Standard Format

JSONL uses newline-delimited JSON objects:

{"id": 1, "name": "Alice", "age": 30}
{"id": 2, "name": "Bob", "age": 25}
{"id": 3, "name": "Charlie", "age": 35}

Advanced Features

Column Selection

Select specific columns using columnNames:

// Only load specific columns
const table = await loadJsonTable({
  data: "data.jsonl",
  format: {
    name: "jsonl",
    columnNames: ["name", "age"],
  },
})

Array Format Handling

Handle CSV-style array data with rowType: "array":

// Input JSONL with arrays:
// ["id", "name"]
// [1, "Alice"]
// [2, "Bob"]

const table = await loadJsonTable({
  data: "data.jsonl",
  format: {
    name: "jsonl",
    rowType: "array",
  },
})

Remote File Loading

// Load from URL
const table = await loadJsonTable({
  data: "https://example.com/data.jsonl",
})

// Load multiple remote files
const table = await loadJsonTable({
  data: [
    "https://api.example.com/logs-2023.jsonl",
    "https://api.example.com/logs-2024.jsonl",
  ],
})

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