langchain-hello-world

LangChain Hello World

Overview

Minimal working examples demonstrating LCEL (LangChain Expression Language) -- the .pipe() chain syntax that is the foundation of all LangChain applications.

Prerequisites

• Completed langchain-install-auth setup
• Valid LLM provider API key configured

Example 1: Simplest Chain (TypeScript)

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

// Three components: prompt -> model -> parser
const prompt = ChatPromptTemplate.fromTemplate("Tell me a joke about {topic}");
const model = new ChatOpenAI({ model: "gpt-4o-mini" });
const parser = new StringOutputParser();

// LCEL: chain them with .pipe()
const chain = prompt.pipe(model).pipe(parser);

const result = await chain.invoke({ topic: "TypeScript" });
console.log(result);
// "Why do TypeScript developers wear glasses? Because they can't C#!"

Example 2: Chat with System Prompt

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

const prompt = ChatPromptTemplate.fromMessages([
  ["system", "You are a {persona}. Keep answers under 50 words."],
  ["human", "{question}"],
]);

const chain = prompt
  .pipe(new ChatOpenAI({ model: "gpt-4o-mini" }))
  .pipe(new StringOutputParser());

const answer = await chain.invoke({
  persona: "senior DevOps engineer",
  question: "What is the most important Kubernetes concept?",
});
console.log(answer);

Example 3: Structured Output with Zod

import { ChatOpenAI } from "@langchain/openai";
import { ChatPromptTemplate } from "@langchain/core/prompts";
import { z } from "zod";

const ReviewSchema = z.object({
  sentiment: z.enum(["positive", "negative", "neutral"]),
  confidence: z.number().min(0).max(1),
  summary: z.string().describe("One-sentence summary"),
});

const model = new ChatOpenAI({ model: "gpt-4o-mini" });
const structuredModel = model.withStructuredOutput(ReviewSchema);

const prompt = ChatPromptTemplate.fromTemplate(
  "Analyze the sentiment of this review:\n\n{review}"
);

const chain = prompt.pipe(structuredModel);

const result = await chain.invoke({
  review: "LangChain makes building AI apps surprisingly straightforward.",
});
console.log(result);
// { sentiment: "positive", confidence: 0.92, summary: "..." }

Example 4: Streaming

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

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

// Stream tokens as they arrive
const stream = await chain.stream({ topic: "coding" });
for await (const chunk of stream) {
  process.stdout.write(chunk);
}

Example 5: Python Equivalent

from langchain_openai import ChatOpenAI
from langchain_core.prompts import ChatPromptTemplate
from langchain_core.output_parsers import StrOutputParser

prompt = ChatPromptTemplate.from_template("Tell me about {topic}")
model = ChatOpenAI(model="gpt-4o-mini")
parser = StrOutputParser()

# LCEL uses | operator in Python
chain = prompt | model | parser

result = chain.invoke({"topic": "LangChain"})
print(result)

How LCEL Works

Every component in an LCEL chain implements the Runnable interface:

Method	Purpose
.invoke(input)	Single input, single output
.batch(inputs)	Process array of inputs
.stream(input)	Yield output chunks
.pipe(next)	Chain to next runnable

The .pipe() method (or | in Python) creates a RunnableSequence where each step's output feeds the next step's input. Every LangChain component -- prompts, models, parsers, retrievers -- is a Runnable.

Error Handling

Error	Cause	Fix
Missing value for input topic	Template variable not in invoke args	Match invoke({}) keys to template {variables}
Cannot read properties of undefined	Chain not awaited	Add await before .invoke()
Rate limit reached	Too many API calls	Add delay or use gpt-4o-mini for testing

Resources

• LCEL Conceptual Guide
• Prompt Templates
• Output Parsers

Next Steps

Proceed to langchain-core-workflow-a for advanced chain composition.