Core: mild mergedeep speedup (#12718)

* Core: mergedeep speedup

uses direct assignment, loops over top level

* Update utils.js

* Update utils.js

* linting

src/utils.js

@@ -1023,38 +1023,51 @@ export function deepEqual(obj1, obj2, {checkTypes = false} = {}) {
 }
 
 export function mergeDeep(target, ...sources) {
-  if (!sources.length) return target;
-  const source = sources.shift();
-
-  if (isPlainObject(target) && isPlainObject(source)) {
-    for (const key in source) {
-      if (isPlainObject(source[key])) {
-        if (!target[key]) Object.assign(target, { [key]: {} });
-        mergeDeep(target[key], source[key]);
-      } else if (isArray(source[key])) {
-        if (!target[key]) {
-          Object.assign(target, { [key]: [...source[key]] });
-        } else if (isArray(target[key])) {
-          source[key].forEach(obj => {
-            let addItFlag = 1;
-            for (let i = 0; i < target[key].length; i++) {
-              if (deepEqual(target[key][i], obj)) {
-                addItFlag = 0;
-                break;
-              }
-            }
-            if (addItFlag) {
-              target[key].push(obj);
-            }
-          });
-        }
+  for (let i = 0; i < sources.length; i++) {
+    const source = sources[i];
+    if (!isPlainObject(source)) {
+      continue;
+    }
+    mergeDeepHelper(target, source);
+  }
+  return target;
+}
+
+function mergeDeepHelper(target, source) {
+  // quick check
+  if (!isPlainObject(target) || !isPlainObject(source)) {
+    return;
+  }
+
+  const keys = Object.keys(source);
+  for (let i = 0; i < keys.length; i++) {
+    const key = keys[i];
+    if (key === '__proto__' || key === 'constructor') {
+      continue;
+    }
+    const val = source[key];
+
+    if (isPlainObject(val)) {
+      if (!target[key]) {
+        target[key] = {};
+      }
+      mergeDeepHelper(target[key], val);
+    } else if (Array.isArray(val)) {
+      if (!Array.isArray(target[key])) {
+        target[key] = [...val];
       } else {
-        Object.assign(target, { [key]: source[key] });
+        // deduplicate
+        val.forEach(obj => {
+          if (!target[key].some(item => deepEqual(item, obj))) {
+            target[key].push(obj);
+          }
+        });
       }
+    } else {
+      // direct assignment
+      target[key] = val;
     }
   }
-
-  return mergeDeep(target, ...sources);
 }
 
 /**
@@ -1245,7 +1258,7 @@ export function hasNonSerializableProperty(obj, checkedObjects = new Set()) {
  *
  * @param {Array} collection - Array of objects.
  * @param {String} key - Key of nested property.
- * @returns {any, undefined} - Value of nested property.
+ * @returns {any|undefined} - Value of nested property.
  */
 export function setOnAny(collection, key) {
   for (let i = 0, result; i < collection.length; i++) {