Using the Fetch API with Undici in Node.js

Introduction

Unidici is an HTTP client libary that powers the fetch API in Node.js. It was written from scratch and does not rely on the built-in HTTP client in Node.js. It includes a number of features that make it a good choice for high-performance applications.

async function main() {
  const response = await fetch('https://jsonplaceholder.typicode.com/posts');
  const data = await response.json();
  console.log(data);
}

main().catch(console.error);

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Customizing the Fetch API with Undici

Undici allows you to customize the Fetch API by providing options to the fetch function. For example, you can set custom headers, set the request method, and set the request body. Here is an example of how you can customize the Fetch API with Undici:

With a pool, you can reuse connections to the same server, which can improve performance. Here is an example of how you can use a pool with Undici:

import { Pool } from 'undici';

const ollamaPool = new Pool('http://localhost:11434', {
  connections: 10,
});

async function streamOllamaCompletion(prompt) {
  const { statusCode, body } = await ollamaPool.request({
    path: '/api/generate',
    method: 'POST',
    headers: {
      'Content-Type': 'application/json',
    },
    body: JSON.stringify({ prompt, model: 'deepseek-r1:8b' }),
  });

  if (statusCode !== 200) {
    throw new Error(`Ollama request failed with status ${statusCode}`);
  }

  let partial = '';

  const decoder = new TextDecoder();
  for await (const chunk of body) {
    partial += decoder.decode(chunk, { stream: true });
    console.log(partial);
  }

  console.log('Streaming complete.');
}

try {
  await streamOllamaCompletion('What is recursion?');
} catch (error) {
  console.error('Error calling Ollama:', error);
} finally {
  console.log('Closing Ollama pool.');
  ollamaPool.close();
}

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Streaming Responses with Undici

import { stream } from 'undici';
import { Writable } from 'stream';

async function fetchGitHubRepos() {
  const url = 'https://api.github.com/users/nodejs/repos';

  const { statusCode } = await stream(
    url,
    {
      method: 'GET',
      headers: {
        'User-Agent': 'undici-stream-example',
        Accept: 'application/json',
      },
    },
    () => {
      let buffer = '';

      return new Writable({
        write(chunk, encoding, callback) {
          buffer += chunk.toString();

          try {
            const json = JSON.parse(buffer);
            console.log(
              'Repository Names:',
              json.map(repo => repo.name)
            );
            buffer = '';
          } catch (error) {
            console.error('Error parsing JSON:', error);
          }

          callback();
        },
        final(callback) {
          console.log('Stream processing completed.');
          callback();
        },
      });
    }
  );

  console.log(`Response status: ${statusCode}`);
}

fetchGitHubRepos().catch(console.error);

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