Unhandled Rejection (TypeError): fetch is not a function in Next.js: Causes and How to Fix

Use Node.js's Built-in Fetch

• Next.js requires a specific setup for server-side environments because the fetch API is native to browsers. To utilize fetch in a server-side rendering context, Node.js starting from version 18 provides built-in fetch support.
• Ensure your Node.js version is 18 or higher. You can verify your version using: \`\`\`shell node -v \`\`\`
• Upgrade your Node.js if necessary, which you can do via a package manager, such as homebrew for macOS: \`\`\`shell brew update brew install node \`\`\`

Polyfilling Fetch for Node.js Prior Version 18

• If upgrading Node.js is not an option, consider using a library like node-fetch to polyfill fetch functionality.
• Install node-fetch using npm or yarn: \`\`\`shell npm install node-fetch \`\`\` or \`\`\`shell yarn add node-fetch \`\`\`
• Import node-fetch at the top of your file where the fetch function is being used, taking care to conditionally apply it only on the server side: \`\`\`javascript import fetch from 'node-fetch';

  if (!globalThis.fetch) {
    globalThis.fetch = fetch;
  }
  \`\`\`
  

Utilize Next.js API Routes

• For server-side operations, consider leveraging Next.js API routes which provide a built-in serverless function setup, eliminating the need to import or polyfill fetch.
• Create an API route within the pages/api directory of your Next.js project, for example: pages/api/my-api.js.
• Use the built-in fetch function directly in those API routes, which will behave as expected because of the serverless nature of Next.js: \`\`\`javascript export default async function handler(req, res) { const response = await fetch('https://api.example.com/data'); const data = await response.json(); res.status(200).json(data); } \`\`\`

Testing and Verification

• Implement thorough testing to ensure fetch is functioning properly, both during development and in production.
• Start your Next.js application using: \`\`\`shell npm run dev \`\`\` and observe console logs or application behavior for successful data fetching to debug any arising issues.
• Use utility libraries like Jest for testing server-side code with mocked fetch, improving code reliability and robustness.