Jest

LangChain.js is a comprehensive TypeScript/JavaScript framework for building applications powered by large language models (LLMs). It provides a unified interface for connecting to 20+ LLM providers (OpenAI, Anthropic Claude, Google Gemini, AWS Bedrock, Ollama, etc.), managing document retrieval with vector stores, and orchestrating AI agents with tools, memory, and middleware capabilities.

Tech Stack

JestPostgreSQLRedisExpress.jsNext.jsReactTypeScriptVitestZod

Key Features

Universal Chat Models
Vector Store Integrations
Document Loaders & Text Splitters
Agent Middleware System
Runnables & Chains
MCP Adapters
Tracing & Observability

Coding Conventions

Standards and patterns used in this codebase

Build & Configuration

Use ESLint with a shared internal configuration package (@langchain/internal/eslint) that extends across all packages in the monorepo

import { config } from "@langchain/internal/eslint";
export default [...config];

examples/eslint.config.tsinternal/standard-tests/eslint.config.tslibs/create-langchain-integration/eslint.config.ts

Build & Configuration

Use TypeScript ESM modules with .ts extension in ESLint config files (eslint.config.ts) following flat config format

export default [
  ...config,
  {
    files: ["src/**/*.ts"],
    rules: {
      "@typescript-eslint/no-explicit-any": "off"
    }
  }
];

examples/eslint.config.tsinternal/eslint/eslint.config.ts

Module System

Use ES module syntax with named exports and dynamic imports for lazy loading

const { run } = await import(`./examples/${exampleRelativePath}`);
await run();

examples/src/index.ts

Error Handling

Use process.exit with appropriate exit codes (0 for success, 1 for errors) and console.error for error output

try {
  await run();
  process.exit(0);
} catch (e) {
  console.error(e);
  process.exit(1);
}

examples/src/index.tsenvironment_tests/scripts/test-runner.ts

CLI & User Input

Use prompts library for interactive CLI input with validation and abort handling via onCancel

const res = await prompts({
  type: "text",
  name: "path",
  message: "What is your project named?",
  validate: (name) => { ... }
}, { onCancel: () => process.exit(1) });

libs/create-langchain-integration/index.ts

File System Operations

Use async fs operations (fs/promises) with path.join for cross-platform path handling

import fs from "fs";
import path from "path";
const root = path.resolve(appPath);
await fs.promises.mkdir(root, { recursive: true });

libs/create-langchain-integration/create-app.ts

Build & Configuration

Configure Vite/webpack to externalize Node.js built-in modules and provide browser fallbacks for server-only modules

resolve: {
  fallback: {
    fs: false,
    child_process: false,
    "node:child_process": false
  }
}

environment_tests/test-exports-vercel/next.config.jsenvironment_tests/test-exports-vite/vite.config.js

Testing

Use Docker containers for isolated environment testing with spawn for process execution and proper signal handling

const proc = spawn("docker", ["compose", "up", "--build", "--abort-on-container-exit"], {
  cwd: testDir,
  stdio: "inherit"
});

environment_tests/scripts/test-runner.ts

Package Structure

Organize monorepo with libs/ for packages, providers/ for LLM integrations, and internal/ for shared tooling

libs/langchain-core/src, libs/providers/langchain-openai/src, internal/eslint/

examples/eslint.config.tslibs/create-langchain-integration/create-app.ts

Naming Conventions

Use kebab-case for package and directory names, camelCase for variables and functions

@langchain/langchain-openai, createApp(), appPath, projectName

libs/create-langchain-integration/create-app.tslibs/create-langchain-integration/index.ts