Flow: Fix generating arrow functions with param (#4504)

* transform-flow-comments, single arrow param support #4503

* further tests for printing single arrow func param flow code

* cleanup

packages/babel-generator/src/generators/methods.js

@@ -77,8 +77,10 @@ export function ArrowFunctionExpression(node: Object) {
     this.space();
   }
 
-  if (node.params.length === 1 && t.isIdentifier(node.params[0])) {
-    this.print(node.params[0], node);
+  const firstParam = node.params[0];
+
+  if (node.params.length === 1 && t.isIdentifier(firstParam) && !hasTypes(node, firstParam)) {
+    this.print(firstParam, node);
   } else {
     this._params(node);
   }
@@ -89,3 +91,7 @@ export function ArrowFunctionExpression(node: Object) {
 
   this.print(node.body, node);
 }
+
+function hasTypes(node, param) {
+  return node.typeParameters || node.returnType || param.typeAnnotation || param.optional || param.trailingComments;
+}

packages/babel-generator/test/fixtures/flow/arrow-functions/actual.js

@@ -0,0 +1,7 @@
+const bar1 = (x: number): string => {};
+const bar2 = (x?) => {};
+const bar3 = (x?: string) => {};
+const bar4 = x => {};
+const bar5 = (x): string => {};
+const bar6 = (x: number) => {};
+const bar7 = <T>(x) => {};

packages/babel-generator/test/fixtures/flow/arrow-functions/expected.js

@@ -0,0 +1,7 @@
+const bar1 = (x: number): string => {};
+const bar2 = (x?) => {};
+const bar3 = (x?: string) => {};
+const bar4 = x => {};
+const bar5 = (x): string => {};
+const bar6 = (x: number) => {};
+const bar7 = <T>(x) => {};

packages/babel-plugin-transform-flow-comments/test/fixtures/flow-comments/arrow-single-optional-param/actual.js

@@ -0,0 +1 @@
+const x = (foo?) => {}

packages/babel-plugin-transform-flow-comments/test/fixtures/flow-comments/arrow-single-optional-param/expected.js

@@ -0,0 +1 @@
+const x = (foo /*:: ?*/) => {};

packages/babel-plugin-transform-flow-comments/test/fixtures/flow-comments/arrow-single-param-type/actual.js

@@ -0,0 +1 @@
+const x = (foo: string) => {};

packages/babel-plugin-transform-flow-comments/test/fixtures/flow-comments/arrow-single-param-type/expected.js

@@ -0,0 +1 @@
+const x = (foo /*: string*/) => {};