From 71b6e683c01fa0737494211c9744e2e6bf8ede66 Mon Sep 17 00:00:00 2001
From: Eoghan Murray <eoghan@getthere.ie>
Date: Wed, 1 Apr 2026 12:00:00 +0800
Subject: [PATCH] Add a `slimDOM` option to strip out unnecessary parts of the
 DOM (#36)

* Add a `slimDOM` option to strip out unnecessary parts of the DOM in terms of replay

 - <script> tags in the <head> take up unnecessary storage space and are often injected semi randomly to become a source of unnecessary variation between recordings of the same thing
 - comment tags can be stripped out without affecting display
 - future: this option could also turn on more aggressive stripping, e.g. elements that are hidden by CSS (assuming we can handle them becoming visible after mutation events)

* Mark nodes ignored due to slimDOM option, so that they can also be ignored by the mutation observer in rrweb

* Introducing the `ignored` attribute violates the `serializedNodeWithId` type

* slimDOM: Strip out whitespace nodes from <head> element as they have no effect but take up space

 - these would otherwise have to be merged after <script> elements are removed; for statcounter usecase, removing
   <script> elements is no good if there is still a trace of their presence due to the white space (and hence a variant <head> node is still produced)
 - I explored a more radical stripping of all white space nodes, but there is a problem if parent node is <pre> or otherwise rendered with `white-space: pre` and similar.
   detecting applied styles with getComputedStyle would be very expensive (I haven't measured it though)

* Export IGNORED_NODE as a constant instead of relying on the hard-to-grok `-2`

* Remove <link rel=preload as=script> which are similarly as useless as <script> tags

* Make slimDOM configurable with the expecations that `slimDOMOptions: true` will only enable non-destructive options (so not all options may be turned on)

* Expand slimDOM to add options to remove more elements from the <head> that should not be necessary in the replayer context
---
 src/index.ts    |   2 +
 src/snapshot.ts | 124 ++++++++++++++++++++++++++++++++++++++++++++++++
 src/types.ts    |  13 +++++
 3 files changed, 139 insertions(+)
diff --git a/src/index.ts b/src/index.ts
index 0688bb24..77ca8377 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -2,6 +2,7 @@ import snapshot, {
   serializeNodeWithId,
   transformAttribute,
   visitSnapshot,
+  IGNORED_NODE,
 } from './snapshot';
 import rebuild, { buildNodeWithSN, addHoverClass } from './rebuild';
 export * from './types';
@@ -14,4 +15,5 @@ export {
   addHoverClass,
   transformAttribute,
   visitSnapshot,
+  IGNORED_NODE,
 };
diff --git a/src/snapshot.ts b/src/snapshot.ts
index fec9813d..17150d0a 100644
--- a/src/snapshot.ts
+++ b/src/snapshot.ts
@@ -6,11 +6,14 @@ import {
   INode,
   idNodeMap,
   MaskInputOptions,
+  SlimDOMOptions,
 } from './types';
 
 let _id = 1;
 const tagNameRegex = RegExp('[^a-z1-6-_]');
 
+export const IGNORED_NODE = -2;
+
 function genId(): number {
   return _id++;
 }
@@ -323,6 +326,83 @@ function serializeNode(
   }
 }
 
+function lowerIfExists(maybeAttr : string | number | boolean) : string {
+  if (maybeAttr === undefined) {
+    return '';
+  } else {
+    return (maybeAttr as string).toLowerCase();
+  }
+}
+
+function slimDOMExcluded(sn: serializedNode, slimDOMOptions: SlimDOMOptions): boolean {
+  if (slimDOMOptions.comment && sn.type === NodeType.Comment) {
+    // TODO: convert IE conditional comments to real nodes
+    return true;
+  } else if (sn.type === NodeType.Element) {
+    if (slimDOMOptions.script &&
+        (sn.tagName === 'script' ||
+         (sn.tagName === 'link' && sn.attributes.rel === 'preload' && sn.attributes['as'] === 'script')
+        )) {
+      return true;
+    } else if (slimDOMOptions.headFavicon && (
+      (sn.tagName === 'link' && sn.attributes.rel === 'shortcut icon')
+        || (sn.tagName === 'meta' && (
+          lowerIfExists(sn.attributes['name']).match(/^msapplication-tile(image|color)$/)
+            || lowerIfExists(sn.attributes['name']) === 'application-name'
+            || lowerIfExists(sn.attributes['rel']) === 'icon'
+            || lowerIfExists(sn.attributes['rel']) === 'apple-touch-icon'
+            || lowerIfExists(sn.attributes['rel']) === 'shortcut icon'
+        )))) {
+      return true;
+    } else if (sn.tagName === 'meta') {
+      if (slimDOMOptions.headMetaDescKeywords && (
+        lowerIfExists(sn.attributes['name']).match(/^description|keywords$/)
+      )) {
+        return true;
+      } else if (slimDOMOptions.headMetaSocial && (
+        lowerIfExists(sn.attributes['property']).match(/^(og|twitter|fb):/)  // og = opengraph (facebook)
+          || lowerIfExists(sn.attributes['name']).match(/^(og|twitter):/)
+          || lowerIfExists(sn.attributes['name']) === 'pinterest'
+      )) {
+        return true;
+      } else if (slimDOMOptions.headMetaRobots && (
+        lowerIfExists(sn.attributes['name']) === 'robots'
+          || lowerIfExists(sn.attributes['name']) === 'googlebot'
+          || lowerIfExists(sn.attributes['name']) === 'bingbot'
+      )) {
+        return true;
+      } else if (slimDOMOptions.headMetaHttpEquiv && (
+        sn.attributes['http-equiv'] !== undefined
+      )) {
+        // e.g. X-UA-Compatible, Content-Type, Content-Language,
+        // cache-control, X-Translated-By
+        return true;
+      } else if (slimDOMOptions.headMetaAuthorship && (
+        lowerIfExists(sn.attributes['name']) === 'author'
+          || lowerIfExists(sn.attributes['name']) === 'generator'
+          || lowerIfExists(sn.attributes['name']) === 'framework'
+          || lowerIfExists(sn.attributes['name']) === 'publisher'
+          || lowerIfExists(sn.attributes['name']) === 'progid'
+          || lowerIfExists(sn.attributes['property']).match(/^article:/)
+          || lowerIfExists(sn.attributes['property']).match(/^product:/)
+      )) {
+        return true;
+      } else if (slimDOMOptions.headMetaVerification && (
+        lowerIfExists(sn.attributes['name']) === 'google-site-verification'
+          || lowerIfExists(sn.attributes['name']) === 'yandex-verification'
+          || lowerIfExists(sn.attributes['name']) === 'csrf-token'
+          || lowerIfExists(sn.attributes['name']) === 'p:domain_verify'
+          || lowerIfExists(sn.attributes['name']) === 'verify-v1'
+          || lowerIfExists(sn.attributes['name']) === 'verification'
+          || lowerIfExists(sn.attributes['name']) === 'shopify-checkout-api-token'
+      )) {
+        return true;
+      }
+    }
+  }
+  return false;
+}
+
 export function serializeNodeWithId(
   n: Node | INode,
   doc: Document,
@@ -331,7 +411,9 @@ export function serializeNodeWithId(
   skipChild = false,
   inlineStylesheet = true,
   maskInputOptions?: MaskInputOptions,
+  slimDOMOptions: SlimDOMOptions = {},
   recordCanvas?: boolean,
+  preserveWhiteSpace = true,
 ): serializedNodeWithId | null {
   const _serializedNode = serializeNode(
     n,
@@ -346,15 +428,26 @@ export function serializeNodeWithId(
     console.warn(n, 'not serialized');
     return null;
   }
+
   let id;
   // Try to reuse the previous id
   if ('__sn' in n) {
     id = n.__sn.id;
+  } else if (slimDOMExcluded(_serializedNode, slimDOMOptions) ||
+             (!preserveWhiteSpace &&
+              _serializedNode.type === NodeType.Text &&
+              !_serializedNode.isStyle &&
+              !_serializedNode.textContent.replace(/^\s+|\s+$/gm,'').length
+             )) {
+    id = IGNORED_NODE;
   } else {
     id = genId();
   }
   const serializedNode = Object.assign(_serializedNode, { id });
   (n as INode).__sn = serializedNode;
+  if (id === IGNORED_NODE) {
+    return null;  // slimDOM
+  }
   map[id] = n as INode;
   let recordChild = !skipChild;
   if (serializedNode.type === NodeType.Element) {
@@ -367,6 +460,14 @@ export function serializeNodeWithId(
       serializedNode.type === NodeType.Element) &&
     recordChild
   ) {
+    if (
+      (slimDOMOptions.headWhitespace &&
+       _serializedNode.type === NodeType.Element &&
+       _serializedNode.tagName == 'head')
+      // would impede performance: || getComputedStyle(n)['white-space'] === 'normal'
+    ) {
+      preserveWhiteSpace = false;
+    }
     for (const childN of Array.from(n.childNodes)) {
       const serializedChildNode = serializeNodeWithId(
         childN,
@@ -376,7 +477,9 @@ export function serializeNodeWithId(
         skipChild,
         inlineStylesheet,
         maskInputOptions,
+        slimDOMOptions,
         recordCanvas,
+        preserveWhiteSpace,
       );
       if (serializedChildNode) {
         serializedNode.childNodes.push(serializedChildNode);
@@ -391,6 +494,7 @@ function snapshot(
   blockClass: string | RegExp = 'rr-block',
   inlineStylesheet = true,
   maskAllInputsOrOptions: boolean | MaskInputOptions,
+  slimDOMSensibleOrOptions: boolean | SlimDOMOptions,
   recordCanvas?: boolean,
 ): [serializedNodeWithId | null, idNodeMap] {
   const idNodeMap: idNodeMap = {};
@@ -416,6 +520,25 @@ function snapshot(
       : maskAllInputsOrOptions === false
       ? {}
       : maskAllInputsOrOptions;
+  const slimDOMOptions: SlimDOMOptions =
+    (slimDOMSensibleOrOptions === true ||
+     slimDOMSensibleOrOptions === 'all')
+  // if true: set of sensible options that should not throw away any information
+    ? {
+      script: true,
+      comment: true,
+      headFavicon: true,
+      headWhitespace: true,
+      headMetaDescKeywords: slimDOMSensibleOrOptions === 'all',  // destructive
+      headMetaSocial: true,
+      headMetaRobots: true,
+      headMetaHttpEquiv: true,
+      headMetaAuthorship: true,
+      headMetaVerification: true,
+    }
+  : slimDOMSensibleOrOptions === false
+    ? {}
+  : slimDOMSensibleOrOptions;
   return [
     serializeNodeWithId(
       n,
@@ -425,6 +548,7 @@ function snapshot(
       false,
       inlineStylesheet,
       maskInputOptions,
+      slimDOMOptions,
       recordCanvas,
     ),
     idNodeMap,
diff --git a/src/types.ts b/src/types.ts
index d8546e84..9a53c012 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -87,3 +87,16 @@ export type MaskInputOptions = Partial<{
   textarea: boolean;
   select: boolean;
 }>;
+
+export type SlimDOMOptions = Partial<{
+  script: boolean;
+  comment: boolean;
+  headFavicon: boolean;
+  headWhitespace: boolean;
+  headMetaDescKeywords: boolean;
+  headMetaSocial: boolean;
+  headMetaRobots: boolean;
+  headMetaHttpEquiv: boolean;
+  headMetaAuthorship: boolean;
+  headMetaVerification: boolean;
+}>;
