first pass at generating feature-list json

scripts/update-bcd.ts

@@ -350,6 +350,12 @@ interface UpdateLog {
   reason: Reason;
 }
 
+interface FeatureListLog {
+  browser: BrowserName;
+  path: string;
+  statements: SimpleSupportStatement[];
+}
+
 /** Values available in operations. */
 interface UpdateState extends UpdateLog {
   shared: UpdateShared;
@@ -1008,6 +1014,18 @@ const pickLog = <T extends UpdateLog>({
   };
 };
 
+const pickFeatureList = <T extends UpdateLog>({
+  browser,
+  path,
+  statements,
+}: T): FeatureListLog => {
+  return {
+    browser,
+    path,
+    statements,
+  };
+};
+
 /**
  * Generates a sequence of key-value pairs representing the entries in an object tree.
  * The keys are generated by concatenating the prefix with each nested key.
@@ -1041,8 +1059,8 @@ export const update = (
   bcd: Identifier,
   supportMatrix: SupportMatrix,
   options: any,
-): boolean => {
-  const changes: UpdateLog[] = [];
+): FeatureListLog[] => {
+  const results: UpdateLog[] = [];
   for (const state of compose(
     expand("entry", function* () {
       for (const [path, entry] of walkEntries("", bcd)) {
@@ -1136,7 +1154,7 @@ export const update = (
       }
     }),
   )()) {
-    changes.push(pickLog(state));
+    results.push(pickLog(state));
     if (state.statements) {
       state.shared.support[state.browser] =
         state.statements.length === 1 ? state.statements[0] : state.statements;
@@ -1145,8 +1163,11 @@ export const update = (
       logger.warn(state.reason.message);
     }
   }
-  // TODO: Serialize changes to a file
-  return changes.some(({statements}) => Boolean(statements));
+
+  const updates = results
+    .filter(({statements}) => Boolean(statements))
+    .map((result) => pickFeatureList(result));
+  return updates;
 };
 
 /* c8 ignore start */
@@ -1232,18 +1253,25 @@ export const main = async (
     browsers,
     overrides.filter(Array.isArray as (item: unknown) => item is OverrideTuple),
   );
+  let featureList: FeatureListLog[] = [];
 
   // Should match https://github.com/mdn/browser-compat-data/blob/f10bf2cc7d1b001a390e70b7854cab9435ffb443/test/linter/test-style.js#L63
   // TODO: https://github.com/mdn/browser-compat-data/issues/3617
   for (const [file, data] of Object.entries(bcdFiles)) {
-    const modified = update(data, supportMatrix, filter);
-    if (!modified) {
+    const updates = update(data, supportMatrix, filter);
+    if (!updates.length) {
       continue;
     }
+    featureList = featureList.concat(updates);
     logger.info(`Updating ${path.relative(BCD_DIR, file)}`);
     const json = JSON.stringify(data, null, "  ") + "\n";
     await fs.writeFile(file, json);
   }
+
+  if (Boolean(featureList.length)) {
+    const featureListJSON = JSON.stringify(featureList, null, "  ") + "\n";
+    await fs.writeFile("feature-list.json", featureListJSON);
+  }
 };
 
 if (esMain(import.meta)) {