fix: Ensure that the webpack output filename ends with .js.

index.js

@@ -76,7 +76,9 @@ const preprocessor = (options = {}) => {
     // we're provided a default output path that lives alongside Cypress's
     // app data files so we don't have to worry about where to put the bundled
     // file on disk
-    const outputPath = file.outputPath
+    const outputPath = path.extname(file.outputPath) === '.js'
+      ? file.outputPath
+      : `${file.outputPath}.js`
 
     // we need to set entry and output
     webpackOptions = Object.assign(webpackOptions, {

test/unit/index.spec.js

@@ -132,6 +132,17 @@ describe('webpack preprocessor', function () {
         })
       })
 
+      it('adds .js extension to filename when the originating file had been no javascript file', function () {
+        this.file.outputPath = 'output/output.ts'
+
+        return this.run().then(() => {
+          expect(webpack.lastCall.args[0].output).to.eql({
+            path: 'output',
+            filename: 'output.ts.js',
+          })
+        })
+      })
+
       it('enables inline source maps', function () {
         return this.run().then(() => {
           expect(webpack.lastCall.args[0].devtool).to.equal('inline-source-map')
@@ -179,6 +190,14 @@ describe('webpack preprocessor', function () {
         })
       })
 
+      it('adds .js extension and resolves with that output path when the originating file had been no javascript file', function () {
+        this.file.outputPath = 'output/output.ts'
+
+        return this.run().then((outputPath) => {
+          expect(outputPath).to.be.equal('output/output.ts.js')
+        })
+      })
+
       it('emits "rerun" when shouldWatch is true and there is an update', function () {
         this.file.shouldWatch = true
         this.compilerApi.watch.yields(null, this.statsApi)