next.js/test/unit/eslint-plugin-next/no-unwanted-polyfillio.test.ts

import { RuleTester } from 'eslint'
import { rules } from '@next/eslint-plugin-next'

const NextESLintRule = rules['no-unwanted-polyfillio']

const tests = {
  valid: [
    `import {Head} from 'next/document';

      export class Blah extends Head {
        render() {
          return (
            <div>
              <h1>Hello title</h1>
              <script src='https://polyfill.io/v3/polyfill.min.js?features=AbortController'></script>
            </div>
          );
        }
    }`,
    `import {Head} from 'next/document';

      export class Blah extends Head {
        render() {
          return (
            <div>
              <h1>Hello title</h1>
              <script src='https://polyfill.io/v3/polyfill.min.js?features=IntersectionObserver'></script>
            </div>
          );
        }
    }`,
    `import Script from 'next/script';

      export function MyApp({ Component, pageProps }) {
          return (
            <div>
              <Component {...pageProps} />
              <Script src='https://polyfill.io/v3/polyfill.min.js?features=IntersectionObserver' />
            </div>
          );
    }`,
    `import Script from 'next/script';

      export function MyApp({ Component, pageProps }) {
          return (
            <div>
              <Component {...pageProps} />
              <Script src='https://polyfill-fastly.io/v3/polyfill.min.js?features=IntersectionObserver' />
            </div>
          );
    }`,
  ],

  invalid: [
    {
      code: `import {Head} from 'next/document';

      export class Blah extends Head {
        render() {
          return (
            <div>
              <h1>Hello title</h1>
              <script src='https://polyfill.io/v3/polyfill.min.js?features=WeakSet%2CPromise%2CPromise.prototype.finally%2Ces2015%2Ces5%2Ces6'></script>
            </div>
          );
        }
    }`,
      errors: [
        {
          message:
            'No duplicate polyfills from Polyfill.io are allowed. WeakSet, Promise, Promise.prototype.finally, es2015, es5, es6 are already shipped with Next.js. See: https://nextjs.org/docs/messages/no-unwanted-polyfillio',
          type: 'JSXOpeningElement',
        },
      ],
    },
    {
      code: `
      export class Blah {
        render() {
          return (
            <div>
              <h1>Hello title</h1>
              <script src='https://polyfill.io/v3/polyfill.min.js?features=Array.prototype.copyWithin'></script>
            </div>
          );
        }
    }`,
      errors: [
        {
          message:
            'No duplicate polyfills from Polyfill.io are allowed. Array.prototype.copyWithin is already shipped with Next.js. See: https://nextjs.org/docs/messages/no-unwanted-polyfillio',
          type: 'JSXOpeningElement',
        },
      ],
    },
    {
      code: `import NextScript from 'next/script';

      export function MyApp({ Component, pageProps }) {
          return (
            <div>
              <Component {...pageProps} />
              <NextScript src='https://polyfill.io/v3/polyfill.min.js?features=Array.prototype.copyWithin' />
            </div>
          );
    }`,
      errors: [
        {
          message:
            'No duplicate polyfills from Polyfill.io are allowed. Array.prototype.copyWithin is already shipped with Next.js. See: https://nextjs.org/docs/messages/no-unwanted-polyfillio',
          type: 'JSXOpeningElement',
        },
      ],
    },
    {
      code: `import {Head} from 'next/document';

        export class ES2019Features extends Head {
          render() {
            return (
              <div>
                <h1>Hello title</h1>
                <script src='https://polyfill.io/v3/polyfill.min.js?features=Object.fromEntries'></script>
              </div>
            );
          }
      }`,
      errors: [
        {
          message:
            'No duplicate polyfills from Polyfill.io are allowed. Object.fromEntries is already shipped with Next.js. See: https://nextjs.org/docs/messages/no-unwanted-polyfillio',
        },
      ],
    },
  ],
}

describe('no-unwanted-polyfillio', () => {
  new RuleTester({
    languageOptions: {
      ecmaVersion: 2018,
      sourceType: 'module',
      parserOptions: {
        ecmaFeatures: {
          modules: true,
          jsx: true,
        },
      },
    },
  }).run('eslint', NextESLintRule, tests)
})