feat: support async formatter

docs/developer-guide/nodejs-api.md

@@ -404,7 +404,7 @@ This edit information means replacing the range of the `range` property by the `
 
 The `Formatter` value is the object to convert the [LintResult] objects to text. The [eslint.loadFormatter()][eslint-loadformatter] method returns it. It has the following method:
 
-* `format` (`(results: LintResult[]) => string`)<br>
+* `format` (`(results: LintResult[]) => string | Promise<string>`)<br>
   The method to convert the [LintResult] objects to text.
 
 ---

docs/developer-guide/working-with-custom-formatters.md

@@ -11,6 +11,16 @@ module.exports = function(results) {
 };
 ```
 
+Formatter can also be an async function (from ESLint 8.2.0), the following shows a simple example:
+
+```js
+//my-awesome-formatter.js
+module.exports = async function(results) {
+    const formatted = await asyncTask();
+    return formatted;
+};
+```
+
 To run ESLint with this formatter, you can use the `-f` (or `--format`) command line flag:
 
 ```bash

lib/cli.js

@@ -178,7 +178,7 @@ async function printResults(engine, results, format, outputFile) {
         return false;
     }
 
-    const output = formatter.format(results);
+    const output = await formatter.format(results);
 
     if (output) {
         if (outputFile) {

lib/eslint/eslint.js

@@ -34,7 +34,12 @@ const { version } = require("../../package.json");
 /** @typedef {import("../shared/types").LintMessage} LintMessage */
 /** @typedef {import("../shared/types").Plugin} Plugin */
 /** @typedef {import("../shared/types").Rule} Rule */
-/** @typedef {import("./load-formatter").Formatter} Formatter */
+
+/**
+ * The main formatter object.
+ * @typedef Formatter
+ * @property {function(LintResult[]): string | Promise<string>} [format] format function.
+ */
 
 /**
  * The options with which to configure the ESLint instance.
@@ -629,7 +634,7 @@ class ESLint {
             /**
              * The main formatter method.
              * @param {LintResults[]} results The lint results to format.
-             * @returns {string} The formatted lint results.
+             * @returns {string | Promise<string>} The formatted lint results.
              */
             format(results) {
                 let rulesMeta = null;

tests/fixtures/formatters/async.js

@@ -0,0 +1,4 @@
+/*global module*/
+module.exports = function(results) {
+  return Promise.resolve('from async formatter');
+};

tests/lib/cli-engine/cli-engine.js

@@ -1160,7 +1160,7 @@ describe("CLIEngine", () => {
 
             const report = engine.executeOnFiles([getFixturePath("formatters")]);
 
-            assert.strictEqual(report.results.length, 3);
+            assert.strictEqual(report.results.length, 4);
             assert.strictEqual(report.errorCount, 0);
             assert.strictEqual(report.warningCount, 0);
             assert.strictEqual(report.fixableErrorCount, 0);
@@ -1200,14 +1200,18 @@ describe("CLIEngine", () => {
             assert.strictEqual(report.results[0].warningCount, 0);
             assert.strictEqual(report.results[0].fixableErrorCount, 0);
             assert.strictEqual(report.results[0].fixableWarningCount, 0);
-            assert.strictEqual(report.results[1].errorCount, 3);
+            assert.strictEqual(report.results[1].errorCount, 0);
             assert.strictEqual(report.results[1].warningCount, 0);
-            assert.strictEqual(report.results[1].fixableErrorCount, 3);
+            assert.strictEqual(report.results[1].fixableErrorCount, 0);
             assert.strictEqual(report.results[1].fixableWarningCount, 0);
             assert.strictEqual(report.results[2].errorCount, 3);
             assert.strictEqual(report.results[2].warningCount, 0);
             assert.strictEqual(report.results[2].fixableErrorCount, 3);
             assert.strictEqual(report.results[2].fixableWarningCount, 0);
+            assert.strictEqual(report.results[3].errorCount, 3);
+            assert.strictEqual(report.results[3].warningCount, 0);
+            assert.strictEqual(report.results[3].fixableErrorCount, 3);
+            assert.strictEqual(report.results[3].fixableWarningCount, 0);
         });
 
         it("should process when file is given by not specifying extensions", () => {

tests/lib/cli.js

@@ -287,6 +287,17 @@ describe("cli", () => {
         });
     });
 
+    describe("when given an async formatter path", () => {
+        it("should execute without any errors", async () => {
+            const formatterPath = getFixturePath("formatters", "async.js");
+            const filePath = getFixturePath("passing.js");
+            const exit = await cli.execute(`-f ${formatterPath} ${filePath}`);
+
+            assert.strictEqual(log.info.getCall(0).args[0], "from async formatter");
+            assert.strictEqual(exit, 0);
+        });
+    });
+
     describe("when executing a file with a lint error", () => {
         it("should exit with error", async () => {
             const filePath = getFixturePath("undef.js");

tests/lib/eslint/eslint.js

@@ -1212,7 +1212,7 @@ describe("ESLint", () => {
             });
             const results = await eslint.lintFiles([getFixturePath("formatters")]);
 
-            assert.strictEqual(results.length, 3);
+            assert.strictEqual(results.length, 4);
             assert.strictEqual(results[0].messages.length, 0);
             assert.strictEqual(results[1].messages.length, 0);
             assert.strictEqual(results[2].messages.length, 0);