Alternate prompt template formats

The primary template format for LangChain prompts is the simple and versatile f-string. LangChain.js supports handlebars as an experimental alternative.

Note that templates created this way cannot be added to the LangChain prompt hub and may have unexpected behavior if you're using tracing.

Setup

You'll need to install the handlebars templating engine package:

npm install handlebars

Usage

import { HandlebarsPromptTemplate } from "langchain/experimental/prompts/handlebars";
import { ChatAnthropic } from "@langchain/anthropic";
import { StringOutputParser } from "@langchain/core/output_parsers";

const template = `Tell me a joke about {{topic}}`;

const prompt = HandlebarsPromptTemplate.fromTemplate(template);

const formattedResult = await prompt.invoke({ topic: "bears" });

console.log(formattedResult);

/*
  StringPromptValue {
    value: 'Tell me a joke about bears'
  }
*/

const model = new ChatAnthropic();

const chain = prompt.pipe(model).pipe(new StringOutputParser());

const result = await chain.invoke({
  topic: "bears",
});

console.log(result);

/*
  Why did the bears dissolve their hockey team? Because there were too many grizzly fights!
*/

API Reference:

• HandlebarsPromptTemplate from langchain/experimental/prompts/handlebars
• ChatAnthropic from @langchain/anthropic
• StringOutputParser from @langchain/core/output_parsers