feat: show an explicit message when undefined is included as a config

Fixes eslint#18259

lib/eslint/eslint.js

@@ -395,6 +395,10 @@ async function calculateConfigArray(eslint, {
  const fileConfig = await loadFlatConfigFile(configFilePath);
 
  if (Array.isArray(fileConfig)) {
+ if (fileConfig.includes(void 0)) {
+ throw new Error("You have included `undefined` in your array of configs - this is commonly the result of trying to use a config from a plugin that does not exist; make sure you have not typo'd the name!");
+ }
+
  configs.push(...fileConfig);
  } else {
  configs.push(fileConfig);

tests/fixtures/undefined-config/a.js

@@ -0,0 +1 @@
+var foo = "bar";

tests/fixtures/undefined-config/eslint.config.js

@@ -0,0 +1,14 @@
+// as in '= require("eslint-plugin-mine")'
+const myPlugin = {
+ configs: { recommended: {} }
+}
+
+module.exports = [
+ {
+ rules: {
+ quotes: ["error", "single"]
+ }
+ },
+ // 'whoops, this should really be "recommended"'
+ myPlugin.configs.eslint_recommended
+]

tests/lib/eslint/eslint.js

@@ -1222,6 +1222,16 @@ describe("ESLint", () => {
  assert.strictEqual(results[0].messages[0].ruleId, "quotes");
  });
 
+ it("should error early with config files that include an undefined", async () => {
+ eslint = new ESLint({
+ cwd: getFixturePath("undefined-config")
+ });
+
+ await assert.rejects(async () => {
+ await eslint.lintFiles(["a*.js"]);
+ }, /You have included `undefined` in your array of configs - this is commonly the result of trying to use a config from a plugin that does not exist; make sure you have not typo'd the name!/u);
+ });
+
  // https://github.com/eslint/eslint/issues/16265
  describe("Dot files in searches", () => {