fix: add better error messaging for builds

package.json

@@ -82,6 +82,7 @@
   "jest": {
     "testMatch": [
       "**/test/**/*.js",
+      "**/test/**/*.ts",
       "!**/test/fixtures/**",
       "!**/test/sample/**"
     ],

plugin/src/helpers/verification.ts

@@ -62,10 +62,18 @@ export const checkNextSiteHasBuilt = ({
   failBuild: FailBuild
 }): void | never => {
   if (!existsSync(path.join(publish, 'BUILD_ID'))) {
-    const outWarning =
-      path.basename(publish) === 'out'
-        ? `Your publish directory is set to "out", but in most cases it should be ".next".`
-        : `In most cases it should be set to ".next", unless you have chosen a custom "distDir" in your Next config.`
+    let outWarning
+
+    if (path.basename(publish) === 'out') {
+      outWarning = `Your publish directory is set to "out", but in most cases it should be ".next".`
+    } else if (path.basename(publish) !== '.next' && existsSync(path.join('.next', 'BUILD_ID'))) {
+      outWarning = outdent`
+        However, a '.next' directory was found with a production build.
+        Consider changing your 'publish' directory to '.next'
+      `
+    } else {
+      outWarning = `In most cases it should be set to ".next", unless you have chosen a custom "distDir" in your Next config.`
+    }
 
     return failBuild(outdent`
       The directory "${publish}" does not contain a Next.js production build. Perhaps the build command was not run, or you specified the wrong publish directory.
@@ -74,7 +82,7 @@ export const checkNextSiteHasBuilt = ({
     `)
   }
   if (existsSync(path.join(publish, 'export-detail.json'))) {
-    failBuild(outdent`
+    return failBuild(outdent`
       Detected that "next export" was run, but site is incorrectly publishing the ".next" directory.
       The publish directory should be set to "out", and you should set the environment variable NETLIFY_NEXT_PLUGIN_SKIP to "true".
     `)

test/helpers/verification.spec.ts

@@ -0,0 +1,80 @@
+import { checkNextSiteHasBuilt } from '../../plugin/src/helpers/verification'
+import { outdent } from 'outdent'
+
+import type { NetlifyPluginUtils } from '@netlify/build'
+type FailBuild = NetlifyPluginUtils['build']['failBuild']
+
+jest.mock('fs', () => {
+  return {
+    existsSync: jest.fn()
+  }
+})
+
+describe('checkNextSiteHasBuilt', () => {
+  let failBuildMock
+  const { existsSync } = require('fs')
+
+  beforeEach(() => {
+    failBuildMock = (jest.fn() as unknown) as FailBuild
+  })
+
+  afterEach(() => {
+    jest.clearAllMocks()
+    jest.resetAllMocks()
+  })
+
+  it('returns error message about incorrectly publishing the ".next" directory when "next export" was run', () => {
+    existsSync.mockReturnValue(true)
+
+    const expectedFailureMessage = outdent`
+    Detected that "next export" was run, but site is incorrectly publishing the ".next" directory.
+    The publish directory should be set to "out", and you should set the environment variable NETLIFY_NEXT_PLUGIN_SKIP to "true".
+    `
+
+    checkNextSiteHasBuilt({ publish: '.next', failBuild: failBuildMock })
+
+    expect(failBuildMock).toHaveBeenCalledWith(expectedFailureMessage)
+  })
+
+  it('returns error message prompt to change publish directory to ".next"', () => {
+    // False for not initially finding the specified 'publish' directory, 
+    // True for successfully finding a '.next' directory with a production build
+    existsSync.mockReturnValueOnce(false).mockReturnValueOnce(true)
+
+    const expectedFailureMessage = outdent`
+    The directory "someCustomDir" does not contain a Next.js production build. Perhaps the build command was not run, or you specified the wrong publish directory.
+    However, a '.next' directory was found with a production build.
+    Consider changing your 'publish' directory to '.next'
+    If you are using "next export" then you should set the environment variable NETLIFY_NEXT_PLUGIN_SKIP to "true".
+  `
+
+    checkNextSiteHasBuilt({ publish: 'someCustomDir', failBuild: failBuildMock })
+
+    expect(failBuildMock).toHaveBeenCalledWith(expectedFailureMessage)
+  })
+
+  it('returns error message prompt when publish directory is set to "out"', () => {
+    existsSync.mockReturnValue(false)
+
+    const expectedFailureMessage = outdent`
+    The directory "out" does not contain a Next.js production build. Perhaps the build command was not run, or you specified the wrong publish directory.
+    Your publish directory is set to "out", but in most cases it should be ".next".
+    If you are using "next export" then you should set the environment variable NETLIFY_NEXT_PLUGIN_SKIP to "true".
+  `
+    checkNextSiteHasBuilt({ publish: 'out', failBuild: failBuildMock })
+
+    expect(failBuildMock).toHaveBeenCalledWith(expectedFailureMessage)
+  })
+
+  it('returns default error message when production build was not found', () => {
+    existsSync.mockReturnValue(false)
+    const expectedFailureMessage = outdent`
+    The directory ".next" does not contain a Next.js production build. Perhaps the build command was not run, or you specified the wrong publish directory.
+    In most cases it should be set to ".next", unless you have chosen a custom "distDir" in your Next config.
+    If you are using "next export" then you should set the environment variable NETLIFY_NEXT_PLUGIN_SKIP to "true".
+  `
+    checkNextSiteHasBuilt({ publish: '.next', failBuild: failBuildMock })
+
+    expect(failBuildMock).toHaveBeenCalledWith(expectedFailureMessage)
+  })
+})