feat: add makeJSOnlyValue to generator API (#3568)

Provides convenience method for passing JS into config files.
Closes issue #3535.

docs/dev-guide/generator-api.md

@@ -97,6 +97,14 @@ Add a message to be printed when the generator exits (after any other standard m
 - **Usage**:
 Convenience method for generating a JS config file from JSON
 
+## makeJSOnlyValue
+
+- **Arguments**
+  - `{any} str` - JS expression as a string
+
+- **Usage**:
+Turns a string expression into executable JS for .js config files
+
 ## injectImports
 
 - **Arguments**

packages/@vue/cli/__tests__/Generator.spec.js

@@ -656,3 +656,23 @@ test('extract config files', async () => {
   expect(fs.readFileSync('/jest.config.js', 'utf-8')).toMatch(js(configs.jest))
   expect(fs.readFileSync('/.browserslistrc', 'utf-8')).toMatch('> 1%\nnot <= IE8')
 })
+
+test('generate a JS-Only value from a string', async () => {
+  const jsAsString = 'true ? "alice" : "bob"'
+
+  const generator = new Generator('/', { plugins: [
+    {
+      id: 'test',
+      apply: api => {
+        api.extendPackage({
+          testScript: api.makeJSOnlyValue(jsAsString)
+        })
+      }
+    }
+  ] })
+
+  await generator.generate({})
+
+  expect(generator.pkg).toHaveProperty('testScript')
+  expect(typeof generator.pkg.testScript).toBe('function')
+})

packages/@vue/cli/lib/GeneratorAPI.js

@@ -240,6 +240,16 @@ class GeneratorAPI {
     return `module.exports = ${stringifyJS(value, null, 2)}`
   }
 
+  /**
+   * Turns a string expression into executable JS for JS configs.
+   * @param {*} str JS expression as a string
+   */
+  makeJSOnlyValue (str) {
+    const fn = () => {}
+    fn.__expression = str
+    return fn
+  }
+
   /**
    * Add import statements to a file.
    */