implement declared class fields

Summary:
Flow implemented class fields while they were very early-stage, behind the `esproposal.class_instance_fields` and `esproposal.class_static_fields` flowconfig options.

In current versions of the proposal, fields without initializers now default to `undefined`: [[design](https://github.com/tc39/proposal-class-fields#fields-without-initializers-are-set-to-undefined)] [[spec](https://tc39.es/proposal-class-fields/#sec-define-field)]

Babel < 8 stripped uninitialized fields, which is not spec-compliant. Babel 8 changes to leaving the field and just removing the annotation, as you'd rightly expect, but making it impossible* to have type-only fields. [[babel issue](babel/babel#10039)] [[babel 8 notes](babel/babel#10746)]

This diff introduces a `declare` syntax to provide a solution for type-only fields:

```
class C {
  declare foo: string;
  bar: string;
}

// becomes
class C {
  bar;
}
```

As far as Flow is concerned, both `foo` and `bar` are treated as before, as if they are `string`:

```
const x = new C;
(x.foo: string);
(x.bar: string);
```

Fixes #6811

[* technically you can use Flow comment syntax -- `class C { /*:: prop: string; */ }` but we don't want to require that]

Reviewed By: samwgoldman

Differential Revision: D20088452

fbshipit-source-id: f00299539c9387a55102e9a4e554a3b3bcb3498e

packages/flow-remove-types/flow-remove-types

@@ -23,6 +23,11 @@ var usage = 'Usage: flow-remove-types [options] [sources] \n' +
 '  -a, --all         Transform all files, not just those with a @flow comment\n' +
 '  -p, --pretty      Remove flow types without replacing with spaces, \n' +
 '                    producing prettier output but may require using source maps\n' +
+'      --ignore-uninitialized-fields\n' +
+'                    Removes uninitialized class fields (`foo;`, `foo: string;`)\n' +
+'                    completely rather than only removing the type. THIS IS NOT\n' +
+'                    SPEC COMPLIANT! Instead, use `declare foo: string;` for\n' +
+'                    type-only fields.\n' +
 '  -m, --sourcemaps  Also output source map files. Optionally pass "inline"\n' +
 '  -q, --quiet       Does not produce any output concerning successful progress.\n' +
 
@@ -79,6 +84,7 @@ var outDir;
 var outFile;
 var all;
 var pretty;
+var ignoreUninitializedFields;
 var sourceMaps;
 var inlineSourceMaps;
 var quiet;
@@ -104,6 +110,8 @@ while (i < process.argv.length) {
     all = true;
   } else if (arg === '-p' || arg === '--pretty') {
     pretty = true;
+  } else if (arg === '--ignore-uninitialized-fields') {
+    ignoreUninitializedFields = true;
   } else if (arg === '-m' || arg === '--sourcemaps') {
     sourceMaps = true;
     if (process.argv[i] === 'inline') {
@@ -242,7 +250,11 @@ function btoa(str) {
 
 function transformSource(content, filepath) {
   try {
-    return flowRemoveTypes(content, { all: all, pretty: pretty });
+    return flowRemoveTypes(content, {
+      all: all,
+      pretty: pretty,
+      ignoreUninitializedFields: ignoreUninitializedFields,
+    });
   } catch (error) {
     if (error.loc) {
       var line = error.loc.line - 1;

packages/flow-remove-types/index.js

@@ -3,6 +3,8 @@
  *
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
+ *
+ * @format
  */
 
 var parse = require('flow-parser').parse;
@@ -21,6 +23,11 @@ var vlq = require('vlq');
  *     If true, removes types completely rather than replacing with spaces.
  *     This may require using source maps.
  *
+ *   - ignoreUninitializedFields: (default: false)
+ *     If true, removes uninitialized class fields (`foo;`, `foo: string;`)
+ *     completely rather than only removing the type. THIS IS NOT SPEC
+ *     COMPLIANT! Instead, use `declare foo: string;` for type-only fields.
+ *
  * Returns an object with two methods:
  *
  *   - .toString()
@@ -34,7 +41,7 @@ module.exports = function flowRemoveTypes(source, options) {
   var all = Boolean(options && options.all);
   if (options && options.checkPragma) {
     throw new Error(
-      'flow-remove-types: the "checkPragma" option has been replaced by "all".'
+      'flow-remove-types: the "checkPragma" option has been replaced by "all".',
     );
   }
 
@@ -68,6 +75,9 @@ module.exports = function flowRemoveTypes(source, options) {
     source: source,
     removedNodes: removedNodes,
     pretty: Boolean(options && options.pretty),
+    ignoreUninitializedFields: Boolean(
+      options && options.ignoreUninitializedFields,
+    ),
   };
 
   // Remove the flow pragma.
@@ -172,7 +182,7 @@ var removeFlowVisitor = {
   },
 
   ClassProperty: function(context, node) {
-    if (!node.value) {
+    if (node.declare || (context.ignoreUninitializedFields && !node.value)) {
       return removeNode(context, node);
     }
   },
@@ -256,8 +266,8 @@ var removeFlowVisitor = {
             context,
             endOf(ast.tokens[paramEndIdx]),
             ast.tokens[paramEndIdx].loc.end,
-            ' =>'
-          )
+            ' =>',
+          ),
         );
 
         // Delete the original arrow token.

packages/flow-remove-types/test-update.sh

@@ -7,6 +7,9 @@
 # Generate expected output
 ./flow-remove-types test/source.js > test/expected.js;
 
+# Generate expected output with --ignore-uninitialized-fields flag
+./flow-remove-types --ignore-uninitialized-fields test/source.js > test/expected-uninitialized-fields.js;
+
 # Generate expected output with --pretty flag
 ./flow-remove-types --pretty test/source.js > test/expected-pretty.js;
 

packages/flow-remove-types/test.sh

@@ -9,6 +9,11 @@ echo "Test: flow-remove-types test/source.js"
 DIFF=$(./flow-remove-types test/source.js | diff test/expected.js -);
 if [ -n "$DIFF" ]; then echo "$DIFF"; exit 1; fi;
 
+# Test expected output with --ignore-uninitialized-fields flag
+echo "Test: flow-remove-types --ignore-uninitialized-fields test/source.js"
+DIFF=$(./flow-remove-types --ignore-uninitialized-fields test/source.js | diff test/expected-uninitialized-fields.js -);
+if [ -n "$DIFF" ]; then echo "$DIFF"; exit 1; fi;
+
 # Test expected output with --pretty flag
 echo "Test: flow-remove-types --pretty test/source.js"
 DIFF=$(./flow-remove-types --pretty test/source.js | diff test/expected-pretty.js -);