Make _getIndexOf() lookup O(1)

Add a property-to-index Map to improve _getIndexOf() lookup times.

Author: asamuzaK

lib/CSSStyleDeclaration.js

@@ -48,6 +48,7 @@ class CSSStyleDeclaration {
 
     // Other internals
     this._length = 0;
+    this._propertyIndices = new Map();
     this._priorities = new Map();
     this._values = new Map();
 
@@ -118,7 +119,7 @@ class CSSStyleDeclaration {
       const name = "NoModificationAllowedError";
       throw new this._global.DOMException(msg, name);
     }
-    this._removeIndexedProperties(0, this._length);
+    this._clearIndexedProperties();
     this._values.clear();
     this._priorities.clear();
     if (this._parentRule || (this._ownerNode && this._updating)) {
@@ -298,7 +299,7 @@ class CSSStyleDeclaration {
     this._priorities.delete(property);
     const index = this._getIndexOf(property);
     if (index >= 0) {
-      this._removeIndexedProperties(index, 1);
+      this._removeIndexedProperty(index);
       if (this._onChange) {
         this._onChange(this.cssText);
       }
@@ -346,25 +347,33 @@ class CSSStyleDeclaration {
   }
 
   /**
-   * Removes indexed properties at the specified index and shifts others.
-   * Removes all indexed properties if count === this._length.
+   * Clears all indexed properties, properties indices and resets length to 0.
    *
    * @private
-   * @param {number} index - The start index.
-   * @param {number} count - The number of properties to remove.
    */
-  _removeIndexedProperties(index, count) {
-    const length = this._length;
-    // Shift subsequent properties.
-    // Will be skipped if count === length.
-    for (let i = index; i < length - count; i++) {
-      this[i] = this[i + count];
-    }
-    // Delete properties at the end.
-    for (let i = length - count; i < length; i++) {
+  _clearIndexedProperties() {
+    this._propertyIndices.clear();
+    for (let i = 0; i < this._length; i++) {
       delete this[i];
     }
-    this._length -= count;
+    this._length = 0;
+  }
+
+  /**
+   * Removes an indexed property at the specified index, shifts others, and updates indices.
+   *
+   * @private
+   * @param {number} index - The index of the property to remove.
+   */
+  _removeIndexedProperty(index) {
+    this._propertyIndices.delete(this[index]);
+    for (let i = index; i < this._length - 1; i++) {
+      const property = this[i + 1];
+      this[i] = property;
+      this._propertyIndices.set(property, i);
+    }
+    delete this[this._length - 1];
+    this._length--;
   }
 
   /**
@@ -375,12 +384,7 @@ class CSSStyleDeclaration {
    * @returns {number} The index of the property, or -1 if not found.
    */
   _getIndexOf(property) {
-    for (let i = 0; i < this._length; i++) {
-      if (this[i] === property) {
-        return i;
-      }
-    }
-    return -1;
+    return this._propertyIndices.get(property) ?? -1;
   }
 
   /**
@@ -406,6 +410,7 @@ class CSSStyleDeclaration {
     if (!this._values.has(property)) {
       // New property.
       this[this._length] = property;
+      this._propertyIndices.set(property, this._length);
       this._length++;
     }
     if (priority === "important") {