feat: Include basePath property in NEXTAUTH_URL when present in config (#1336)

* feat: handle custom base paths when creating NEXTAUTH_URL

* style: lint

Author: ericapisani

plugin/src/index.ts

@@ -79,7 +79,8 @@ const plugin: NetlifyPlugin = {
       console.log(`NextAuth package detected, setting NEXTAUTH_URL environment variable to ${process.env.URL}`)
 
       const config = await getRequiredServerFiles(publish)
-      config.config.env.NEXTAUTH_URL = process.env.URL
+      const nextAuthUrl = `${process.env.URL}${basePath}`
+      config.config.env.NEXTAUTH_URL = nextAuthUrl
 
       await updateRequiredServerFiles(publish, config)
     }

test/index.js

@@ -25,7 +25,7 @@ const {
   patchNextFiles,
   unpatchNextFiles,
 } = require('../plugin/src/helpers/files')
-const { getRequiredServerFiles } = require('../plugin/src/helpers/config')
+const { getRequiredServerFiles, updateRequiredServerFiles } = require('../plugin/src/helpers/config')
 const { dirname } = require('path')
 const { getProblematicUserRewrites } = require('../plugin/src/helpers/verification')
 
@@ -241,6 +241,26 @@ describe('onBuild()', () => {
     delete process.env.URL
   })
 
+  test('includes the basePath on NEXTAUTH_URL when present', async () => {
+    const mockSiteUrl = 'https://my-netlify-site.app'
+    process.env.URL = mockSiteUrl
+
+    await moveNextDist()
+    
+    const initialConfig = await getRequiredServerFiles(netlifyConfig.build.publish)
+    initialConfig.config.basePath = "/foo"
+    await updateRequiredServerFiles(netlifyConfig.build.publish, initialConfig)
+    
+    await plugin.onBuild(defaultArgs)
+
+    expect(onBuildHasRun(netlifyConfig)).toBe(true)
+    const config = await getRequiredServerFiles(netlifyConfig.build.publish)
+
+    expect(config.config.env.NEXTAUTH_URL).toEqual(`${mockSiteUrl}/foo`)
+
+    delete process.env.URL
+  })
+
   test('skips setting NEXTAUTH_URL when next-auth package is not found', async () => {
     isNextAuthInstalled.mockImplementation(() => {
       return false