diff --git a/.eslintrc.js b/.eslintrc.js
index 07ddcba1a3..5ada2d03a7 100644
--- a/.eslintrc.js
+++ b/.eslintrc.js
@@ -8,7 +8,6 @@ const legacyCode = {
   'no-continue': 'off',
   'no-else-return': 'off',
   'no-plusplus': 'off',
-  'no-prototype-builtins': 'off',
   'no-restricted-syntax': 'off',
   'no-this-before-super': 'off',
 };
@@ -71,12 +70,9 @@ module.exports = {
       files: [
         'client/src/entrypoints/admin/comments.js',
         'client/src/entrypoints/admin/core.js',
-        'client/src/entrypoints/admin/expanding-formset.js',
-        'client/src/entrypoints/admin/filtered-select.js',
         'client/src/entrypoints/admin/page-chooser.js',
         'client/src/entrypoints/admin/page-editor.js',
         'client/src/entrypoints/admin/telepath/widgets.js',
-        'client/src/entrypoints/contrib/table_block/table.js',
         'client/src/entrypoints/contrib/typed_table_block/typed_table_block.js',
         'client/src/entrypoints/images/image-chooser-modal.js',
         'client/src/utils/actions.ts',
diff --git a/CHANGELOG.txt b/CHANGELOG.txt
index 480655a95b..db11c1262a 100644
--- a/CHANGELOG.txt
+++ b/CHANGELOG.txt
@@ -22,6 +22,7 @@ Changelog
  * Clean up some unused utility classes and migrate `unlist` to Tailwind utility class `w-list-none` (Loveth Omokaro)
  * Ensure that the `rebuild_references_index` command can run without console output if called with `--verbosity 0` (Omerzahid Ali, Aman Pandey)
  * Add development (contributing to Wagtail) documentation notes for development on Windows (Akua Dokua Asiedu)
+ * Clean up linting on legacy code and add shared util `hasOwn` in TypeScript (Loveth Omokaro)
  * Fix: Make sure workflow timeline icons are visible in high-contrast mode (Loveth Omokaro)
  * Fix: Ensure authentication forms (login, password reset) have a visible border in Windows high-contrast mode (Loveth Omokaro)
  * Fix: Ensure visual consistency between buttons and links as buttons in Windows high-contrast mode (Albina Starykova)
diff --git a/client/src/components/StreamField/blocks/StreamBlock.js b/client/src/components/StreamField/blocks/StreamBlock.js
index 82d57808b9..9ba0773588 100644
--- a/client/src/components/StreamField/blocks/StreamBlock.js
+++ b/client/src/components/StreamField/blocks/StreamBlock.js
@@ -8,6 +8,7 @@ import {
   BaseInsertionControl,
 } from './BaseSequenceBlock';
 import { escapeHtml as h } from '../../../utils/text';
+import { hasOwn } from '../../../utils/hasOwn';
 
 /* global $ */
 
@@ -318,7 +319,7 @@ export class StreamBlock extends BaseSequenceBlock {
     // Check if there are any block types that have count limits
     this.disabledBlockTypes = new Set();
     for (const blockType in this.blockDef.meta.blockCounts) {
-      if (this.blockDef.meta.blockCounts.hasOwnProperty(blockType)) {
+      if (hasOwn(this.blockDef.meta.blockCounts, blockType)) {
         const maxNum = this.getBlockMax(blockType);
 
         if (typeof maxNum === 'number') {
@@ -459,7 +460,7 @@ export class StreamBlock extends BaseSequenceBlock {
     // Block errors
 
     for (const blockIndex in error.blockErrors) {
-      if (error.blockErrors.hasOwnProperty(blockIndex)) {
+      if (hasOwn(error.blockErrors, blockIndex)) {
         this.children[blockIndex].setError(error.blockErrors[blockIndex]);
       }
     }
diff --git a/client/src/components/StreamField/blocks/StructBlock.js b/client/src/components/StreamField/blocks/StructBlock.js
index e4af0c8564..b092ba42eb 100644
--- a/client/src/components/StreamField/blocks/StructBlock.js
+++ b/client/src/components/StreamField/blocks/StructBlock.js
@@ -1,6 +1,7 @@
 /* global $ */
 
 import { escapeHtml as h } from '../../../utils/text';
+import { hasOwn } from '../../../utils/hasOwn';
 
 export class StructBlockValidationError {
   constructor(blockErrors) {
@@ -96,7 +97,7 @@ export class StructBlock {
     const error = errorList[0];
 
     for (const blockName in error.blockErrors) {
-      if (error.blockErrors.hasOwnProperty(blockName)) {
+      if (hasOwn(error.blockErrors, blockName)) {
         this.childBlocks[blockName].setError(error.blockErrors[blockName]);
       }
     }
diff --git a/client/src/entrypoints/admin/expanding-formset.js b/client/src/entrypoints/admin/expanding-formset.js
index 8d5cd43879..a4e987b211 100644
--- a/client/src/entrypoints/admin/expanding-formset.js
+++ b/client/src/entrypoints/admin/expanding-formset.js
@@ -7,7 +7,7 @@ function buildExpandingFormset(prefix, opts = {}) {
   let formCount = parseInt(totalFormsInput.val(), 10);
 
   if (opts.onInit) {
-    for (let i = 0; i < formCount; i++) {
+    for (let i = 0; i < formCount; i += 1) {
       opts.onInit(i);
     }
   }
@@ -29,7 +29,7 @@ function buildExpandingFormset(prefix, opts = {}) {
     if (opts.onAdd) opts.onAdd(formCount);
     if (opts.onInit) opts.onInit(formCount);
 
-    formCount++;
+    formCount += 1;
     totalFormsInput.val(formCount);
   });
 }
diff --git a/client/src/entrypoints/admin/filtered-select.js b/client/src/entrypoints/admin/filtered-select.js
index 3ab49a3a35..e83ebb41cb 100644
--- a/client/src/entrypoints/admin/filtered-select.js
+++ b/client/src/entrypoints/admin/filtered-select.js
@@ -61,7 +61,7 @@ $(() => {
         filteredValues = optionData;
       } else {
         filteredValues = [];
-        for (let i = 0; i < optionData.length; i++) {
+        for (let i = 0; i < optionData.length; i += 1) {
           if (
             optionData[i].value === '' ||
             optionData[i].filterValue.indexOf(chosenFilter) !== -1
@@ -72,7 +72,7 @@ $(() => {
       }
 
       let foundValue = false;
-      for (let i = 0; i < filteredValues.length; i++) {
+      for (let i = 0; i < filteredValues.length; i += 1) {
         const option = $('<option>');
         option.attr('value', filteredValues[i].value);
         if (filteredValues[i].value === currentValue) foundValue = true;
diff --git a/client/src/entrypoints/contrib/table_block/table.js b/client/src/entrypoints/contrib/table_block/table.js
index bf9661d427..6f2dd5e060 100644
--- a/client/src/entrypoints/contrib/table_block/table.js
+++ b/client/src/entrypoints/contrib/table_block/table.js
@@ -3,6 +3,7 @@
 /* global Handsontable */
 
 import $ from 'jquery';
+import { hasOwn } from '../../../utils/hasOwn';
 
 function initTable(id, tableOptions) {
   const containerId = id + '-handsontable-container';
@@ -54,24 +55,21 @@ function initTable(id, tableOptions) {
   }
 
   if (dataForForm !== null) {
-    if (dataForForm.hasOwnProperty('first_row_is_table_header')) {
+    if (hasOwn(dataForForm, 'first_row_is_table_header')) {
       tableHeaderCheckbox.prop(
         'checked',
         dataForForm.first_row_is_table_header,
       );
     }
-    if (dataForForm.hasOwnProperty('first_col_is_header')) {
+    if (hasOwn(dataForForm, 'first_col_is_header')) {
       colHeaderCheckbox.prop('checked', dataForForm.first_col_is_header);
     }
-    if (dataForForm.hasOwnProperty('table_caption')) {
+    if (hasOwn(dataForForm, 'table_caption')) {
       tableCaption.prop('value', dataForForm.table_caption);
     }
   }
 
-  if (
-    !tableOptions.hasOwnProperty('width') ||
-    !tableOptions.hasOwnProperty('height')
-  ) {
+  if (hasOwn(!tableOptions, 'width') || hasOwn(!tableOptions, 'height')) {
     // Size to parent .sequence-member-inner width if width is not given in tableOptions
     $(window).on('resize', () => {
       hot.updateSettings({
@@ -85,8 +83,8 @@ function initTable(id, tableOptions) {
   const getCellsClassnames = function () {
     const meta = hot.getCellsMeta();
     const cellsClassnames = [];
-    for (let i = 0; i < meta.length; i++) {
-      if (meta[i].hasOwnProperty('className')) {
+    for (let i = 0; i < meta.length; i += 1) {
+      if (hasOwn(meta[i], 'className')) {
         cellsClassnames.push({
           row: meta[i].row,
           col: meta[i].col,
@@ -159,10 +157,10 @@ function initTable(id, tableOptions) {
 
   if (dataForForm !== null) {
     // Overrides default value from tableOptions (if given) with value from database
-    if (dataForForm.hasOwnProperty('data')) {
+    if (hasOwn(dataForForm, 'data')) {
       defaultOptions.data = dataForForm.data;
     }
-    if (dataForForm.hasOwnProperty('cell')) {
+    if (hasOwn(dataForForm, 'cell')) {
       defaultOptions.cell = dataForForm.cell;
     }
   }
diff --git a/docs/releases/4.2.md b/docs/releases/4.2.md
index 10832bc962..ea5ec385a7 100644
--- a/docs/releases/4.2.md
+++ b/docs/releases/4.2.md
@@ -31,6 +31,7 @@ depth: 1
  * Clean up some unused utility classes and migrate `unlist` to Tailwind utility class `w-list-none` (Loveth Omokaro)
  * Ensure that the `rebuild_references_index` command can run without console output if called with `--verbosity 0` (Omerzahid Ali, Aman Pandey)
  * Add development (contributing to Wagtail) documentation notes for [development on Windows](development_on_windows) (Akua Dokua Asiedu)
+ * Clean up linting on legacy code and add shared util `hasOwn` in TypeScript (Loveth Omokaro)
 
 ### Bug fixes