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perf: Add property-to-index Map for O(1) lookups in _getIndexOf #288

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perf: Add property-to-index Map for O(1) lookups in _getIndexOf #288

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22 changes: 13 additions & 9 deletions lib/CSSStyleDeclaration.js
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Original file line number Diff line number Diff line change
Expand Up @@ -50,6 +50,7 @@ class CSSStyleDeclaration {
this._length = 0;
this._priorities = new Map();
this._values = new Map();
this._propertyIndices = new Map(); // Maps property name -> index for O(1) lookups

if (context) {
if (typeof context.getComputedStyle === "function") {
Expand Down Expand Up @@ -121,6 +122,7 @@ class CSSStyleDeclaration {
this._removeIndexedProperties(0, this._length);
this._values.clear();
this._priorities.clear();
this._propertyIndices.clear();
if (this._parentRule || (this._ownerNode && this._updating)) {
return;
}
Expand Down Expand Up @@ -355,10 +357,15 @@ class CSSStyleDeclaration {
*/
_removeIndexedProperties(index, count) {
const length = this._length;
// Shift subsequent properties.
// Will be skipped if count === length.
// Remove the properties being deleted from the index map
for (let i = index; i < index + count && i < length; i++) {
this._propertyIndices.delete(this[i]);
}
// Shift subsequent properties and update their indices
for (let i = index; i < length - count; i++) {
this[i] = this[i + count];
const prop = this[i + count];
this[i] = prop;
this._propertyIndices.set(prop, i);
}
// Delete properties at the end.
for (let i = length - count; i < length; i++) {
Expand All @@ -375,12 +382,8 @@ 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;
const index = this._propertyIndices.get(property);
return index !== undefined ? index : -1;
}

/**
Expand All @@ -406,6 +409,7 @@ class CSSStyleDeclaration {
if (!this._values.has(property)) {
// New property.
this[this._length] = property;
this._propertyIndices.set(property, this._length);
this._length++;
}
if (priority === "important") {
Expand Down