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*   [Get started](/v0.1/docs/get_started/)
    
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    *   [Get started](/v0.1/docs/expression_language/get_started/)
    *   [Why use LCEL?](/v0.1/docs/expression_language/why/)
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*   Extending LangChain.js

Extending LangChain.js
======================

Extending LangChain's base abstractions, whether you're planning to contribute back to the open-source repo or build a bespoke internal integration, is encouraged.

Check out these guides for building your own custom classes for the following modules:

*   [Chat models](/v0.1/docs/modules/model_io/chat/custom_chat/) for interfacing with chat-tuned language models.
*   [LLMs](/v0.1/docs/modules/model_io/llms/custom_llm/) for interfacing with text language models.
*   [Output parsers](/v0.1/docs/modules/model_io/output_parsers/custom/) for handling language model outputs.
*   [Retrievers](/v0.1/docs/modules/data_connection/retrievers/custom/) for fetching context from external data sources.
*   [Vectorstores](/v0.1/docs/modules/data_connection/vectorstores/custom/) for interacting with vector databases.
*   [Agents](/v0.1/docs/modules/agents/how_to/custom_agent/) that allow the language model to make decisions autonomously.
*   [Chat histories](/v0.1/docs/modules/memory/chat_messages/custom/) which enable memory in the form of persistent storage of chat messages and sessions.

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