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*   Fake LLM

On this page

Fake LLM
========

LangChain provides a fake LLM chat model for testing purposes. This allows you to mock out calls to the LLM and and simulate what would happen if the LLM responded in a certain way.

Usage[​](#usage "Direct link to Usage")
---------------------------------------

    import { FakeListChatModel } from "@langchain/core/utils/testing";import { HumanMessage } from "@langchain/core/messages";import { StringOutputParser } from "@langchain/core/output_parsers";/** * The FakeListChatModel can be used to simulate ordered predefined responses. */const chat = new FakeListChatModel({  responses: ["I'll callback later.", "You 'console' them!"],});const firstMessage = new HumanMessage("You want to hear a JavasSript joke?");const secondMessage = new HumanMessage(  "How do you cheer up a JavaScript developer?");const firstResponse = await chat.invoke([firstMessage]);const secondResponse = await chat.invoke([secondMessage]);console.log({ firstResponse });console.log({ secondResponse });/** * The FakeListChatModel can also be used to simulate streamed responses. */const stream = await chat  .pipe(new StringOutputParser())  .stream(`You want to hear a JavasSript joke?`);const chunks = [];for await (const chunk of stream) {  chunks.push(chunk);}console.log(chunks.join(""));/** * The FakeListChatModel can also be used to simulate delays in either either synchronous or streamed responses. */const slowChat = new FakeListChatModel({  responses: ["Because Oct 31 equals Dec 25", "You 'console' them!"],  sleep: 1000,});const thirdMessage = new HumanMessage(  "Why do programmers always mix up Halloween and Christmas?");const slowResponse = await slowChat.invoke([thirdMessage]);console.log({ slowResponse });const slowStream = await slowChat  .pipe(new StringOutputParser())  .stream("How do you cheer up a JavaScript developer?");const slowChunks = [];for await (const chunk of slowStream) {  slowChunks.push(chunk);}console.log(slowChunks.join(""));

#### API Reference:

*   [FakeListChatModel](https://api.js.langchain.com/classes/langchain_core_utils_testing.FakeListChatModel.html) from `@langchain/core/utils/testing`
*   [HumanMessage](https://api.js.langchain.com/classes/langchain_core_messages.HumanMessage.html) from `@langchain/core/messages`
*   [StringOutputParser](https://api.js.langchain.com/classes/langchain_core_output_parsers.StringOutputParser.html) from `@langchain/core/output_parsers`

* * *

#### Help us out by providing feedback on this documentation page:

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