feat: add item.getLangChainHandler() to link dataset runs to langchain invocations on JS

[marcklingen]

Discussed in https://github.com/orgs/langfuse/discussions/1969

^{Originally posted by sahellebusch May 3, 2024}
I'm working w/ a typescript application and want to write a test suite using a dataset. I want to reuse some of my application code, which uses the built in langchain tracing mechanism. Is there a similar integration to the python dataset integration called out in the docs?

Example code:

const langfuseConfig = {
    secretKey: process.env.LANGFUSE_SECRET_KEY,
    publicKey: process.env.LANGFUSE_PUBLIC_KEY,
    baseUrl: 'https://us.cloud.langfuse.com',
  };

  const langfuse = new Langfuse(langfuseConfig);

  const prompt = await langfuse.getPrompt('my-prompt');

  const openAI = new ChatOpenAI({
    openAIApiKey: process.env.OPENAI_API_KEY,
    modelName: prompt.config.modelName,
    temperature: prompt.config.temperature,
  });

  const dataset = await langfuse.getDataset('my-dataset');

  await Promise.all(
    dataset.items.map(async i => {
      const res = await RunnableSequence.from([
        ChatPromptTemplate.fromMessages([
          ['system', prompt.getLangchainPrompt()],
          ['user', '{input}'],
        ]),
        openAI,
        StructuredOutputParser.fromZodSchema(
          z.object({ someProperty: z.string() }),
        ),
      ]).invoke(
        { input: i.input },
        {
          callbacks: [
            new CallbackHandler({
              ...langfuseConfig,
              userId: 'someUuid',
              metadata: {
                some: 'metadata',
              },
            }),
          ],
        },
      );

      // Here I want to link and add a score.
    }),