index.js

@@ -3,6 +3,7 @@
 const assureArray = require("assure-array");
 const isPlainObj = require("is-plain-obj");
 const defaultValue = require("default-value");
+const flatten = require("flatten");
 
 const matchSpecial = require("@validatem/match-special");
 const isPlainObjectValidator = require("@validatem/is-plain-object");
@@ -13,7 +14,8 @@ const hasShape = require("@validatem/has-shape");
 module.exports = function normalizeRules(rules, options) {
 	let normalizeObjects = defaultValue(options.normalizeObjects, false);
 
-	let flattened = assureArray(rules).flat();
+	// TODO: Switch to `Array#flat` once Node 10.x goes EOL (April 2021)
+	let flattened = flatten(assureArray(rules));
 	let actualRules = flattened.filter((rule) => rule != null);
 
 	if (normalizeObjects) {
@@ -32,7 +34,7 @@ module.exports = function normalizeRules(rules, options) {
 			}
 		});
 
-		return mapped.flat();
+		return flatten(mapped);
 	} else {
 		return actualRules;
 	}

package.json

@@ -12,6 +12,7 @@
     "@validatem/match-special": "^0.1.0",
     "assure-array": "^1.0.0",
     "default-value": "^1.0.0",
+    "flatten": "^1.0.3",
     "is-plain-obj": "^2.1.0"
   },
   "devDependencies": {

yarn.lock

@@ -513,6 +513,11 @@ flatted@^2.0.0:
   resolved "https://registry.yarnpkg.com/flatted/-/flatted-2.0.2.tgz#4575b21e2bcee7434aa9be662f4b7b5f9c2b5138"
   integrity sha512-r5wGx7YeOwNWNlCA0wQ86zKyDLMQr+/RB8xy74M4hTphfmjlijTSSXGuH8rnvKZnfT9i+75zmd8jcKdMR4O6jA==
 
+flatten@^1.0.3:
+  version "1.0.3"
+  resolved "https://registry.yarnpkg.com/flatten/-/flatten-1.0.3.tgz#c1283ac9f27b368abc1e36d1ff7b04501a30356b"
+  integrity sha512-dVsPA/UwQ8+2uoFe5GHtiBMu48dWLTdsuEd7CKGlZlD78r1TTWBvDuFaFGKCo/ZfEr95Uk56vZoX86OsHkUeIg==
+
 fs.realpath@^1.0.0:
   version "1.0.0"
   resolved "https://registry.yarnpkg.com/fs.realpath/-/fs.realpath-1.0.0.tgz#1504ad2523158caa40db4a2787cb01411994ea4f"