LangChain JS setup: npm packages, Node 22, and the split from the old single package

TL;DR: On JavaScript you install langchain plus @langchain/core, and each model provider is its own scoped package. The runtime needs Node.js 22 or newer. Log it; it tells you which doc page to read.

The fix

1. Do not import model classes from the bare langchain package. They live in @langchain/openai, @langchain/anthropic, and friends.
2. Keep the key in the environment (OPENAIAPIKEY, ANTHROPICAPIKEY). The classes read it from there by default.
3. Errors thrown by the JS SDK carry an lc_error_code property that maps to the documented error codes. Log it; it tells you which doc page to read.

When to use this

• This covers exactly what the title says: LangChain JS setup.
• You are setting this up for the first time, or auditing an existing setup.
• You want the key gotchas in one place before you start.

When not to use this

• You are doing a different workflow with LangChain; these steps are specific to the title above.
• You need the full reference docs; this is the short path, not the manual.

Compatibility

• Not pinned to a specific version; follows current LangChain behavior.

Verified against the official LangChain docs on 2026-09-26. Install: https://docs.langchain.com/oss/javascript/langchain/install ; common errors: https://docs.langchain.com/oss/javascript/common-errors