LangChain LCEL and Retrieval Cheat Sheet

This quick reference guide summarizes LCEL operators and RAG components.

1. LCEL Composition Syntax Reference

• Pipe Operator Method: .pipe(nextComponent) connects steps in sequence.
• RunnablePassthrough: Passes input values through unmodified to subsequent steps.
• RunnableMap / Plain Object: Invokes sub-steps in parallel, returning a unified dictionary object.

2. Document Processing Classes Reference

3. Basic LCEL Chain Invocation Template

import { ChatOpenAI } from "@langchain/openai";
import { ChatPromptTemplate } from "@langchain/core/prompts";
import { StringOutputParser } from "@langchain/core/output_parsers";

const chain = ChatPromptTemplate.fromTemplate("Tell a joke about {topic}")
  .pipe(new ChatOpenAI({ modelName: "gpt-4o-mini" }))
  .pipe(new StringOutputParser());

const reply = await chain.invoke({ topic: "coding" });