Add support for NextJS 404.html page

For Netlify (deployed), the 404.html page in the out/ folder already 
works. For netlify dev, the 404.html page must be at the directory root.

Fixes: #2

Author: FinnWoelm

README.md

@@ -107,6 +107,7 @@ Lastly, add the following lines to your `.gitignore`:
 # Files generated by next-on-netlify command
 functions/nextRouter
 out/
+404.html
 ```
 
 Now you're all set.

cypress/integration/default_spec.js

@@ -199,3 +199,17 @@ describe('dynamic static page', () => {
     cy.get('p').should('contain', 'it has a dynamic URL parameter: /static/:id.')
   })
 })
+
+describe('404 page', () => {
+  it('renders', () => {
+    cy.request({
+      url: '/this-page-does-not-exist',
+      failOnStatusCode: false
+    }).then(response => {
+      expect(response.status).to.eq(404)
+      cy.state('document').write(response.body)
+    })
+
+    cy.get('h2').should('contain', 'This page could not be found.')
+  })
+})

next-on-netlify.js

@@ -182,5 +182,14 @@ writeFileSync(
 )
 
 
+// 6. Copy 404.html to directory root
+// This is a temporary workaround. Currenly, Netlify (deployed) expects the
+// 404.html file in the publish folder, while netlify-cli dev expects it at
+// the directory root. In order to cover both cases, we copy the 404.html to
+// both locations until this is fixed.
+if(existsSync(join(OUTPUT_PATH, "404.html")))
+  copySync(join(OUTPUT_PATH, "404.html"), join(".", "404.html"))
+
+
 // Done!
 console.log("\x1b[1m✅ Success! All done!\x1b[22m")

tests/defaults.test.js

@@ -186,6 +186,19 @@ describe('Static Pages', () => {
   })
 })
 
+describe('404 Page', () => {
+  test('copies 404.html to output directory', () => {
+    const OUTPUT_PATH = join(PROJECT_PATH, "out")
+
+    expect(existsSync(join(OUTPUT_PATH, "404.html"))).toBe(true)
+  })
+
+  // This is required for 404.html to work on netlify-dev
+  test('copies 404.html to directory root', () => {
+    expect(existsSync(join(PROJECT_PATH, "404.html"))).toBe(true)
+  })
+})
+
 describe('Public assets', () => {
   test('copies public files to output directory', () => {
     const OUTPUT_PATH = join(PROJECT_PATH, "out")