Nested Chains in LangChain JS – Passing the Output from One Chain to Another

DanielJune 18, 2024July 26, 2024

In a recent project, I needed LangChain to use the output of one chain as the input for another chain. I wanted to achieve this in the JavaScript version of LangChain.

Here's an example:

const country = executeChain(
    `Which country has the highest population in Latin America?`
)

const fact = executeSecondChain(
    `Tell me the {someFact} about {country}`
)

The idea was to pass the output from the first chain to the second chain along with another parameter.

While trying to Google my way out of this situation, I discovered some Python solutions but nothing JavaScript-related.

It turns out that to nest multiple chains in LangChain.js, you use the RunnableSequence.

Below is the full code:

import { ChatOpenAI } from "@langchain/openai"
import { StringOutputParser } from "@langchain/core/output_parsers"
import { PromptTemplate } from "@langchain/core/prompts"
import { RunnableSequence } from "@langchain/core/runnables"
import * as dotenv from "dotenv"

dotenv.config()

const model = new ChatOpenAI()

const prompt1 = PromptTemplate.fromTemplate(
    `Which country has the highest population in Latin America? 
    Respond only with the name of the country.`
)

const chain1 = prompt1
                .pipe(model)
                .pipe(new StringOutputParser())

const prompt2 = PromptTemplate.fromTemplate(
    `Tell me the {word} of {country}?`
)

const chain2 = prompt2
                .pipe(model)
                .pipe(new StringOutputParser())

const completeChain = RunnableSequence.from([
    {
        country: chain1, 
        word: input => input.word
    },
    chain2
])

const response = await completeChain.invoke({word: 'currency'})
console.log(response)

It will output something like this:

The currency of Brazil is the Brazilian Real, symbolized as BRL.

If you want to give it a test you can pull down the Node.js example for this article from my GitHub.