Add support for function in next.config.js

The configuration for NextJS in next.config.js can also be supplied as
a function (in addition to an object). This commit adds support for
reading the distDir option when the NextJS config is set by a function.

See: #25

Author: FinnWoelm

lib/getNextDistDir.js

@@ -1,26 +1,13 @@
 // Get the NextJS distDir specified in next.config.js
-const { pathExistsSync }  = require('fs-extra')
-const { resolve, join }   = require('path')
-
-// The default dist dir used by NextJS, if not specified otherwise by user
-const DEFAULT_DIST_DIR = join(".", ".next")
+const { PHASE_PRODUCTION_BUILD }  = require('next/constants')
+const { default: loadConfig }     = require('next/dist/next-server/server/config')
+const { resolve, join }           = require('path')
 
 const getNextDistDir = ({ nextConfigPath }) => {
-  // If next.config.js does not exists, default to NextJS' default distDir
-  hasNextConfig = pathExistsSync(nextConfigPath)
-  if(!hasNextConfig)
-    return DEFAULT_DIST_DIR
-
-  // Read next.config.js
-  const resolvedNextConfigPath = resolve(".", nextConfigPath)
-  const nextConfig = require(resolvedNextConfigPath)
-
-  // If distDir is not set, default to NextJS' default distDir
-  const hasDistDir = 'distDir' in nextConfig
-  if(!hasDistDir)
-    return DEFAULT_DIST_DIR
+  // Load next.config.js
+  // Use same code as https://github.com/vercel/next.js/blob/25488f4a03db30cade4d086ba49cd9a50a2ac02e/packages/next/build/index.ts#L114
+  const nextConfig = loadConfig(PHASE_PRODUCTION_BUILD, resolve("."))
 
-  // Return distDir specified by user
   return join(".", nextConfig.distDir)
 }
 

package.json

@@ -31,6 +31,9 @@
     "cypress:local:testonly": "env CYPRESS_SKIP_DEPLOY=true npm run cypress:local",
     "cypress:netlify:testonly": "env CYPRESS_SKIP_DEPLOY=true npm run cypress:netlify"
   },
+  "peerDependencies": {
+    "next": "9.x"
+  },
   "devDependencies": {
     "cypress": "^4.9.0",
     "jest": "^26.1.0",

tests/fixtures/next.config.js-with-function.js

@@ -0,0 +1,15 @@
+const { PHASE_PRODUCTION_BUILD } = require('next/constants')
+
+module.exports = (phase, { defaultConfig }) => {
+  // next-on-netlify uses settings from PHASE_PRODUCTION_BUILD
+  // This is the same phase that is used when running `next build`
+  if (phase === PHASE_PRODUCTION_BUILD) {
+    return {
+      target: 'serverless',
+      distDir: '.myCustomDir'
+    }
+  }
+
+  // Default options
+  return {}
+}

tests/nextConfigFunction.test.js

@@ -0,0 +1,64 @@
+// Test next-on-netlify when config is set from a function in next.config.js
+// See: https://github.com/FinnWoelm/next-on-netlify/issues/25
+const { parse, join } = require('path')
+const { copySync, emptyDirSync, existsSync,
+        readdirSync, readFileSync, readJsonSync } = require('fs-extra')
+const npmRunBuild = require("./helpers/npmRunBuild")
+
+// The name of this test file (without extension)
+const FILENAME = parse(__filename).name
+
+// The directory which will be used for testing.
+// We simulate a NextJS app within that directory, with pages, and a
+// package.json file.
+const PROJECT_PATH = join(__dirname, "builds", FILENAME)
+
+// The directory that contains the fixtures, such as NextJS pages,
+// NextJS config, and package.json
+const FIXTURE_PATH = join(__dirname, "fixtures")
+
+// Capture the output of `npm run build` to verify successful build
+let BUILD_OUTPUT
+
+beforeAll(
+  async () => {
+    // Clear project directory
+    emptyDirSync(PROJECT_PATH)
+    emptyDirSync(join(PROJECT_PATH, "pages"))
+
+    // Copy NextJS pages and config
+    copySync(
+      join(FIXTURE_PATH, "pages"),
+      join(PROJECT_PATH, "pages")
+    )
+    copySync(
+      join(FIXTURE_PATH, "next.config.js-with-function.js"),
+      join(PROJECT_PATH, "next.config.js")
+    )
+
+    // Copy package.json
+    copySync(
+      join(FIXTURE_PATH, "package.json"),
+      join(PROJECT_PATH, "package.json")
+    )
+
+    // Invoke `npm run build`: Build Next and run next-on-netlify
+    const { stdout } = await npmRunBuild({ directory: PROJECT_PATH })
+    BUILD_OUTPUT = stdout
+  },
+  // time out after 180 seconds
+  180 * 1000
+)
+
+describe('Next', () => {
+  test('builds successfully', () => {
+    // NextJS output
+    expect(BUILD_OUTPUT).toMatch("Creating an optimized production build...")
+    expect(BUILD_OUTPUT).toMatch("Automatically optimizing pages...")
+    expect(BUILD_OUTPUT).toMatch("First Load JS shared by all")
+
+    // Next on Netlify output
+    expect(BUILD_OUTPUT).toMatch("Next on Netlify")
+    expect(BUILD_OUTPUT).toMatch("Success! All done!")
+  })
+})