Using the Fetch API with Undici in Node.js

Introduction

In this guide, we will learn how to use the Fetch API with Undici in Node.js. The Fetch API is a modern interface that allows you to make HTTP requests in the browser. It is similar to the XMLHttpRequest object, but with a more powerful and flexible feature set. Undici is a high-performance HTTP/1.1 client that is designed to be used with Node.js. It is built on top of the HTTP/1.1 parser from Node.js core, and it provides a simple and efficient API for making HTTP requests.

Why use Undici?

Undici is a high-performance HTTP client for Node.js. While Node.js 18+ ships with a built-in Fetch API.

import { fetch } from 'undici';

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:

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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