fix: validate ignorePatterns constructor option in FlatESLint class

lib/eslint/eslint-helpers.js

@@ -758,6 +758,9 @@ function processOptions({
     if (typeof ignore !== "boolean") {
         errors.push("'ignore' must be a boolean.");
     }
+    if (!isNonEmptyString(ignorePatterns) && !isArrayOfNonEmptyString(ignorePatterns) && ignorePatterns !== null) {
+        errors.push("'ignorePatterns' must be a non-empty string, an array of non-empty strings, or null.");
+    }
     if (typeof overrideConfig !== "object") {
         errors.push("'overrideConfig' must be an object or null.");
     }

lib/eslint/flat-eslint.js

@@ -76,7 +76,7 @@ const LintResultCache = require("../cli-engine/lint-result-cache");
  * @property {string[]} [fixTypes] Array of rule types to apply fixes for.
  * @property {boolean} [globInputPaths] Set to false to skip glob resolution of input file paths to lint (default: true). If false, each input file paths is assumed to be a non-glob path to an existing file.
  * @property {boolean} [ignore] False disables all ignore patterns except for the default ones.
- * @property {string[]} [ignorePatterns] Ignore file patterns to use in addition to config ignores.
+ * @property {string|string[]} [ignorePatterns] Ignore file patterns to use in addition to config ignores. These patterns are relative to `cwd`.
  * @property {ConfigData} [overrideConfig] Override config object, overrides all configs used with this instance
  * @property {boolean|string} [overrideConfigFile] Searches for default config file when falsy;
  *      doesn't do any config file lookup when `true`; considered to be a config filename

tests/lib/eslint/flat-eslint.js

@@ -182,6 +182,7 @@ describe("FlatESLint", () => {
                     fixTypes: ["xyz"],
                     globInputPaths: "",
                     ignore: "",
+                    ignorePatterns: "",
                     overrideConfig: "",
                     overrideConfigFile: "",
                     plugins: "",
@@ -199,6 +200,7 @@ describe("FlatESLint", () => {
                     "- 'fixTypes' must be an array of any of \"directive\", \"problem\", \"suggestion\", and \"layout\".",
                     "- 'globInputPaths' must be a boolean.",
                     "- 'ignore' must be a boolean.",
+                    "- 'ignorePatterns' must be a non-empty string, an array of non-empty strings, or null.",
                     "- 'overrideConfig' must be an object or null.",
                     "- 'overrideConfigFile' must be a non-empty string, null, or true.",
                     "- 'plugins' must be an object or null.",
@@ -207,6 +209,33 @@ describe("FlatESLint", () => {
             );
         });
 
+        it("should throw readable messages if 'ignorePatterns' is not a non-empty string or an array of non-empty strings.", () => {
+            const invalidIgnorePatterns = [
+                () => {},
+                false,
+                {},
+                "",
+                [[]],
+                [() => {}],
+                [false],
+                [{}],
+                [""],
+                ["foo", ""],
+                ["foo", "", "bar"],
+                ["foo", false, "bar"]
+            ];
+
+            invalidIgnorePatterns.forEach(ignorePatterns => {
+                assert.throws(
+                    () => new FlatESLint({ ignorePatterns }),
+                    new RegExp(escapeStringRegExp([
+                        "Invalid Options:",
+                        "- 'ignorePatterns' must be a non-empty string, an array of non-empty strings, or null."
+                    ].join("\n")), "u")
+                );
+            });
+        });
+
         it("should throw readable messages if 'plugins' option contains empty key", () => {
             assert.throws(
                 () => new FlatESLint({