feat: do not write undefined fields to config files (#3643)

closes #3058

packages/@vue/cli/lib/util/__tests__/extendJSConfig.spec.js

@@ -79,3 +79,29 @@ module.exports = {
 }`
   )
 })
+
+test(`add a new undefined property`, () => {
+  const value = {
+    foo: undefined
+  }
+  const source = `module.exports = {
+  bar: 123
+}`
+
+  expect(extend(value, source)).toMatch(source)
+})
+
+test(`change an existing property to undefined`, () => {
+  const value = {
+    foo: undefined
+  }
+  const source = `module.exports = {
+  foo: 123,
+  bar: 456
+}`
+  expect(extend(value, source)).toMatch(
+    `module.exports = {
+  bar: 456
+}`
+  )
+})

packages/@vue/cli/lib/util/extendJSConfig.js

@@ -45,13 +45,21 @@ module.exports = function extendJSConfig (value, source) {
     const props = valueAST.program.body[0].expression.properties
     const existingProps = node.properties
     for (const prop of props) {
+      const isUndefinedProp =
+        prop.value.type === 'Identifier' && prop.value.name === 'undefined'
+
       const existing = existingProps.findIndex(p => {
         return !p.computed && p.key.name === prop.key.name
       })
       if (existing > -1) {
         // replace
         existingProps[existing].value = prop.value
-      } else {
+
+        // remove `undefined` props
+        if (isUndefinedProp) {
+          existingProps.splice(existing, 1)
+        }
+      } else if (!isUndefinedProp) {
         // append
         existingProps.push(prop)
       }