diff --git a/src/content-linter/lib/helpers/print-annotations.js b/src/content-linter/lib/helpers/print-annotations.ts
similarity index 78%
rename from src/content-linter/lib/helpers/print-annotations.js
rename to src/content-linter/lib/helpers/print-annotations.ts
index 08e1c8fca5fa..0374d7a5a45a 100644
--- a/src/content-linter/lib/helpers/print-annotations.js
+++ b/src/content-linter/lib/helpers/print-annotations.ts
@@ -6,11 +6,13 @@
  */
 
 export function printAnnotationResults(
-  results,
+  // Using 'any' type as results structure is dynamic and comes from various linting tools with different formats
+  results: any,
   { skippableRules = [], skippableFlawProperties = [] } = {},
 ) {
   for (const [file, flaws] of Object.entries(results)) {
-    for (const flaw of flaws) {
+    // Using 'any' type for flaws as they have varying structures depending on the linting rule
+    for (const flaw of flaws as any) {
       if (intersection(flaw.ruleNames, skippableRules)) {
         continue
       }
@@ -52,6 +54,7 @@ export function printAnnotationResults(
   }
 }
 
-function intersection(arr1, arr2) {
-  return arr1.some((item) => arr2.includes(item))
+// Using 'any' types for generic array intersection utility function
+function intersection(arr1: any[], arr2: any[]) {
+  return arr1.some((item: any) => arr2.includes(item))
 }
diff --git a/src/content-linter/lib/linting-rules/frontmatter-schema.js b/src/content-linter/lib/linting-rules/frontmatter-schema.ts
similarity index 77%
rename from src/content-linter/lib/linting-rules/frontmatter-schema.js
rename to src/content-linter/lib/linting-rules/frontmatter-schema.ts
index aa8c331e5bb7..dc67da8449da 100644
--- a/src/content-linter/lib/linting-rules/frontmatter-schema.js
+++ b/src/content-linter/lib/linting-rules/frontmatter-schema.ts
@@ -1,3 +1,4 @@
+// @ts-ignore - markdownlint-rule-helpers doesn't have TypeScript declarations
 import { addError } from 'markdownlint-rule-helpers'
 import { intersection } from 'lodash-es'
 
@@ -5,12 +6,13 @@ import { getFrontmatter } from '../helpers/utils'
 import { formatAjvErrors } from '../helpers/schema-utils'
 import { frontmatter, deprecatedProperties } from '@/frame/lib/frontmatter'
 import readFrontmatter from '@/frame/lib/read-frontmatter'
+import type { RuleParams, RuleErrorCallback, Rule } from '../../types'
 
-export const frontmatterSchema = {
+export const frontmatterSchema: Rule = {
   names: ['GHD012', 'frontmatter-schema'],
   description: 'Frontmatter must conform to the schema',
   tags: ['frontmatter', 'schema'],
-  function: (params, onError) => {
+  function: (params: RuleParams, onError: RuleErrorCallback) => {
     const fm = getFrontmatter(params.lines)
     if (!fm) return
 
@@ -22,25 +24,25 @@ export const frontmatterSchema = {
     for (const key of deprecatedKeys) {
       // Early access articles are allowed to have deprecated properties
       if (params.name.includes('early-access')) continue
-      const line = params.lines.find((line) => line.trim().startsWith(key))
-      const lineNumber = params.lines.indexOf(line) + 1
+      const line = params.lines.find((line: string) => line.trim().startsWith(key))
+      const lineNumber = params.lines.indexOf(line!) + 1
       addError(
         onError,
         lineNumber,
         `The frontmatter property '${key}' is deprecated. Please remove the property from your article's frontmatter.`,
-        line,
-        [1, line.length],
+        line!,
+        [1, line!.length],
         null, // No fix possible
       )
     }
 
     // Check that the frontmatter matches the schema
     const { errors } = readFrontmatter(params.lines.join('\n'), { schema: frontmatter.schema })
-    const formattedErrors = formatAjvErrors(errors)
+    const formattedErrors = formatAjvErrors(errors as any)
     for (const error of formattedErrors) {
       // If the missing property is at the top level, we don't have a line
       // to point to. In that case, the error will be added to line 1.
-      const query = (line) => line.trim().startsWith(`${error.searchProperty}:`)
+      const query = (line: string) => line.trim().startsWith(`${error.searchProperty}:`)
       const line = error.searchProperty === '' ? null : params.lines.find(query)
       const lineNumber = line ? params.lines.indexOf(line) + 1 : 1
       addError(
diff --git a/src/content-linter/lib/linting-rules/image-alt-text-exclude-start-words.js b/src/content-linter/lib/linting-rules/image-alt-text-exclude-start-words.ts
similarity index 77%
rename from src/content-linter/lib/linting-rules/image-alt-text-exclude-start-words.js
rename to src/content-linter/lib/linting-rules/image-alt-text-exclude-start-words.ts
index 9e2bc4358829..769232a5e090 100644
--- a/src/content-linter/lib/linting-rules/image-alt-text-exclude-start-words.js
+++ b/src/content-linter/lib/linting-rules/image-alt-text-exclude-start-words.ts
@@ -1,6 +1,8 @@
+// @ts-ignore - markdownlint-rule-helpers doesn't have TypeScript declarations
 import { addError } from 'markdownlint-rule-helpers'
 
 import { forEachInlineChild, getRange } from '../helpers/utils'
+import type { RuleParams, RuleErrorCallback, MarkdownToken, Rule } from '../../types'
 
 const excludeStartWords = ['image', 'graphic']
 
@@ -8,21 +10,21 @@ const excludeStartWords = ['image', 'graphic']
   Images should have meaningful alternative text (alt text)
   and should not begin with words like "image" or "graphic".
  */
-export const imageAltTextExcludeStartWords = {
+export const imageAltTextExcludeStartWords: Rule = {
   names: ['GHD031', 'image-alt-text-exclude-words'],
   description: 'Alternate text for images should not begin with words like "image" or "graphic"',
   tags: ['accessibility', 'images'],
   parser: 'markdownit',
-  function: (params, onError) => {
-    forEachInlineChild(params, 'image', function forToken(token) {
-      const imageAltText = token.content.trim()
-
+  function: (params: RuleParams, onError: RuleErrorCallback) => {
+    forEachInlineChild(params, 'image', function forToken(token: MarkdownToken) {
       // If the alt text is empty, there is nothing to check and you can't
       // produce a valid range.
       // We can safely return early because the image-alt-text-length rule
       // will fail this one.
       if (!token.content) return
 
+      const imageAltText = token.content.trim()
+
       const range = getRange(token.line, imageAltText)
       if (
         excludeStartWords.some((excludeWord) => imageAltText.toLowerCase().startsWith(excludeWord))
diff --git a/src/content-linter/lib/linting-rules/internal-links-no-lang.js b/src/content-linter/lib/linting-rules/internal-links-no-lang.ts
similarity index 63%
rename from src/content-linter/lib/linting-rules/internal-links-no-lang.js
rename to src/content-linter/lib/linting-rules/internal-links-no-lang.ts
index e6746eb8c99d..d6f318aa43e1 100644
--- a/src/content-linter/lib/linting-rules/internal-links-no-lang.js
+++ b/src/content-linter/lib/linting-rules/internal-links-no-lang.ts
@@ -1,15 +1,18 @@
+// @ts-ignore - markdownlint-rule-helpers doesn't have TypeScript declarations
 import { filterTokens } from 'markdownlint-rule-helpers'
 
 import { addFixErrorDetail, getRange } from '../helpers/utils'
 import { allLanguageKeys } from '@/languages/lib/languages'
+import type { RuleParams, RuleErrorCallback, Rule } from '../../types'
 
-export const internalLinksNoLang = {
+export const internalLinksNoLang: Rule = {
   names: ['GHD002', 'internal-links-no-lang'],
   description: 'Internal links must not have a hardcoded language code',
   tags: ['links', 'url'],
   parser: 'markdownit',
-  function: (params, onError) => {
-    filterTokens(params, 'inline', (token) => {
+  function: (params: RuleParams, onError: RuleErrorCallback) => {
+    // Using 'any' type for token as markdownlint-rule-helpers doesn't provide TypeScript types
+    filterTokens(params, 'inline', (token: any) => {
       for (const child of token.children) {
         if (child.type !== 'link_open') continue
 
@@ -18,14 +21,17 @@ export const internalLinksNoLang = {
         //  ['href', 'get-started'], ['target', '_blank'],
         //  ['rel', 'canonical'],
         // ]
+        // Attribute arrays are tuples of [attributeName, attributeValue] from markdownit parser
         const hrefsMissingSlashes = child.attrs
           // The attribute could also be `target` or `rel`
-          .filter((attr) => attr[0] === 'href')
-          .filter((attr) => attr[1].startsWith('/') || !attr[1].startsWith('//'))
+          .filter((attr: [string, string]) => attr[0] === 'href')
+          .filter((attr: [string, string]) => attr[1].startsWith('/') || !attr[1].startsWith('//'))
           // Filter out link paths that start with language code
-          .filter((attr) => allLanguageKeys.some((lang) => attr[1].split('/')[1] === lang))
+          .filter((attr: [string, string]) =>
+            allLanguageKeys.some((lang) => attr[1].split('/')[1] === lang),
+          )
           // Get the link path from the attribute
-          .map((attr) => attr[1])
+          .map((attr: [string, string]) => attr[1])
         // Create errors for each link path that includes a language code
         for (const linkPath of hrefsMissingSlashes) {
           const range = getRange(child.line, linkPath)
diff --git a/src/content-linter/lib/linting-rules/internal-links-slash.js b/src/content-linter/lib/linting-rules/internal-links-slash.ts
similarity index 66%
rename from src/content-linter/lib/linting-rules/internal-links-slash.js
rename to src/content-linter/lib/linting-rules/internal-links-slash.ts
index 57fe0748e910..39e632e1991b 100644
--- a/src/content-linter/lib/linting-rules/internal-links-slash.js
+++ b/src/content-linter/lib/linting-rules/internal-links-slash.ts
@@ -1,14 +1,17 @@
+// @ts-ignore - markdownlint-rule-helpers doesn't have TypeScript declarations
 import { filterTokens } from 'markdownlint-rule-helpers'
 
 import { addFixErrorDetail, getRange } from '../helpers/utils'
+import type { RuleParams, RuleErrorCallback, Rule } from '../../types'
 
-export const internalLinksSlash = {
+export const internalLinksSlash: Rule = {
   names: ['GHD003', 'internal-links-slash'],
   description: 'Internal links must start with a /',
   tags: ['links', 'url'],
   parser: 'markdownit',
-  function: (params, onError) => {
-    filterTokens(params, 'inline', (token) => {
+  function: (params: RuleParams, onError: RuleErrorCallback) => {
+    // Using 'any' type for token as markdownlint-rule-helpers doesn't provide TypeScript types
+    filterTokens(params, 'inline', (token: any) => {
       for (const child of token.children) {
         if (child.type !== 'link_open') continue
 
@@ -17,20 +20,21 @@ export const internalLinksSlash = {
         //  ['href', '/get-started'], ['target', '_blank'],
         //  ['rel', 'canonical'],
         // ]
+        // Attribute arrays are tuples of [attributeName, attributeValue] from markdownit parser
         const hrefsMissingSlashes = child.attrs
           // The attribute could also be `target` or `rel`
-          .filter((attr) => attr[0] === 'href')
+          .filter((attr: [string, string]) => attr[0] === 'href')
           // Filter out prefixes we don't want to check
           .filter(
-            (attr) =>
+            (attr: [string, string]) =>
               !['http', 'mailto', '#', '/'].some((ignorePrefix) =>
                 attr[1].startsWith(ignorePrefix),
               ),
           )
           // We can ignore empty links because MD042 from markdownlint catches empty links
-          .filter((attr) => attr[1] !== '')
+          .filter((attr: [string, string]) => attr[1] !== '')
           // Get the link path from the attribute
-          .map((attr) => attr[1])
+          .map((attr: [string, string]) => attr[1])
 
         // Create errors for each link path that doesn't start with a /
         for (const linkPath of hrefsMissingSlashes) {
diff --git a/src/content-linter/lib/linting-rules/link-punctuation.ts b/src/content-linter/lib/linting-rules/link-punctuation.ts
index 778a92782ade..37fb46d00ca6 100644
--- a/src/content-linter/lib/linting-rules/link-punctuation.ts
+++ b/src/content-linter/lib/linting-rules/link-punctuation.ts
@@ -1,15 +1,16 @@
 // @ts-ignore - markdownlint-rule-helpers doesn't have TypeScript declarations
 import { addError, filterTokens } from 'markdownlint-rule-helpers'
-import type { RuleParams, RuleErrorCallback } from '../../types'
+import type { RuleParams, RuleErrorCallback, Rule } from '../../types'
 
 import { doesStringEndWithPeriod, getRange, isStringQuoted } from '../helpers/utils'
 
-export const linkPunctuation = {
+export const linkPunctuation: Rule = {
   names: ['GHD001', 'link-punctuation'],
   description: 'Internal link titles must not contain punctuation',
   tags: ['links', 'url'],
   parser: 'markdownit',
   function: (params: RuleParams, onError: RuleErrorCallback) => {
+    // Using 'any' type for token as markdownlint-rule-helpers doesn't provide TypeScript types
     filterTokens(params, 'inline', (token: any) => {
       const { children, line } = token
       let inLink = false
diff --git a/src/content-linter/lib/linting-rules/liquid-quoted-conditional-arg.js b/src/content-linter/lib/linting-rules/liquid-quoted-conditional-arg.ts
similarity index 70%
rename from src/content-linter/lib/linting-rules/liquid-quoted-conditional-arg.js
rename to src/content-linter/lib/linting-rules/liquid-quoted-conditional-arg.ts
index bb47dda2bbc9..45e25552a7a5 100644
--- a/src/content-linter/lib/linting-rules/liquid-quoted-conditional-arg.js
+++ b/src/content-linter/lib/linting-rules/liquid-quoted-conditional-arg.ts
@@ -1,8 +1,10 @@
 import { TokenKind } from 'liquidjs'
+// @ts-ignore - markdownlint-rule-helpers doesn't have TypeScript declarations
 import { addError } from 'markdownlint-rule-helpers'
 
 import { getLiquidTokens, conditionalTags, getPositionData } from '../helpers/liquid-utils'
 import { isStringQuoted } from '../helpers/utils'
+import type { RuleParams, RuleErrorCallback, Rule } from '../../types'
 
 /*
   Checks for instances where a Liquid conditional tag's argument is
@@ -12,18 +14,20 @@ import { isStringQuoted } from '../helpers/utils'
   {% if "foo" %}
   {% ifversion "bar" %}
 */
-export const liquidQuotedConditionalArg = {
+export const liquidQuotedConditionalArg: Rule = {
   names: ['GHD016', 'liquid-quoted-conditional-arg'],
   description: 'Liquid conditional tags should not quote the conditional argument',
   tags: ['liquid', 'format'],
-  function: (params, onError) => {
+  function: (params: RuleParams, onError: RuleErrorCallback) => {
     const content = params.lines.join('\n')
+    // Using 'any' type for tokens as getLiquidTokens returns tokens from liquid-utils.js which lacks type definitions
     const tokens = getLiquidTokens(content)
-      .filter((token) => token.kind === TokenKind.Tag)
-      .filter((token) => conditionalTags.includes(token.name))
-      .filter((token) => {
+      .filter((token: any) => token.kind === TokenKind.Tag)
+      .filter((token: any) => conditionalTags.includes(token.name))
+      .filter((token: any) => {
         const tokensArray = token.args.split(/\s+/g)
-        if (tokensArray.some((arg) => isStringQuoted(arg))) return true
+        // Using 'any' for args as they come from the untyped liquid token structure
+        if (tokensArray.some((arg: any) => isStringQuoted(arg))) return true
         return false
       })
 
diff --git a/src/content-linter/lib/linting-rules/octicon-aria-labels.js b/src/content-linter/lib/linting-rules/octicon-aria-labels.ts
similarity index 79%
rename from src/content-linter/lib/linting-rules/octicon-aria-labels.js
rename to src/content-linter/lib/linting-rules/octicon-aria-labels.ts
index 243167e9cde2..8d42e6a590dd 100644
--- a/src/content-linter/lib/linting-rules/octicon-aria-labels.js
+++ b/src/content-linter/lib/linting-rules/octicon-aria-labels.ts
@@ -1,6 +1,7 @@
 import { TokenKind } from 'liquidjs'
 import { getLiquidTokens, getPositionData } from '../helpers/liquid-utils'
 import { addFixErrorDetail } from '../helpers/utils'
+import type { RuleParams, RuleErrorCallback, Rule } from '../../types'
 /*
 Octicons should always have an aria-label attribute even if aria hidden. For example:
 
@@ -9,20 +10,21 @@ Octicons should always have an aria-label attribute even if aria hidden. For exa
   {% octicon "alert" aria-label="alert" aria-hidden="true" %}
   {% octicon "alert" aria-label="alert" aria-hidden="true" class="foo" %}
 
-  This is necessary for copilot to be able to recognize the svgs correctly when using our API. 
+  This is necessary for copilot to be able to recognize the svgs correctly when using our API.
 
 */
 
-export const octiconAriaLabels = {
+export const octiconAriaLabels: Rule = {
   names: ['GHD044', 'octicon-aria-labels'],
   description: 'Octicons should always have an aria-label attribute even if aria-hidden.',
   tags: ['accessibility', 'octicons'],
   parser: 'markdownit',
-  function: (params, onError) => {
+  function: (params: RuleParams, onError: RuleErrorCallback) => {
     const content = params.lines.join('\n')
+    // Using 'any' type for tokens as getLiquidTokens returns tokens from liquid-utils.js which lacks type definitions
     const tokens = getLiquidTokens(content)
-      .filter((token) => token.kind === TokenKind.Tag)
-      .filter((token) => token.name === 'octicon')
+      .filter((token: any) => token.kind === TokenKind.Tag)
+      .filter((token: any) => token.name === 'octicon')
 
     for (const token of tokens) {
       const { lineNumber, column, length } = getPositionData(token, params.lines)
diff --git a/src/content-linter/tests/unit/code-fence-line-length.js b/src/content-linter/tests/unit/code-fence-line-length.ts
similarity index 100%
rename from src/content-linter/tests/unit/code-fence-line-length.js
rename to src/content-linter/tests/unit/code-fence-line-length.ts
diff --git a/src/content-linter/tests/unit/early-access-references.js b/src/content-linter/tests/unit/early-access-references.ts
similarity index 97%
rename from src/content-linter/tests/unit/early-access-references.js
rename to src/content-linter/tests/unit/early-access-references.ts
index 00a8bf6bcc5b..cb6209e80094 100644
--- a/src/content-linter/tests/unit/early-access-references.js
+++ b/src/content-linter/tests/unit/early-access-references.ts
@@ -41,7 +41,7 @@ describe(frontmatterEarlyAccessReferences.names.join(' - '), () => {
     expect(lineNumbers.includes(4)).toBe(false)
     expect(lineNumbers.includes(5)).toBe(false)
     expect(errors[0].errorRange).toEqual([8, 12])
-    expect(errors[1].errorRange).toEqual([15, 12], [28, 12])
+    expect(errors[1].errorRange).toEqual([15, 12])
   })
   test('early access file with early access references passes', async () => {
     const result = await runRule(frontmatterEarlyAccessReferences, {
diff --git a/src/content-linter/tests/unit/frontmatter-video-transcripts.js b/src/content-linter/tests/unit/frontmatter-video-transcripts.ts
similarity index 100%
rename from src/content-linter/tests/unit/frontmatter-video-transcripts.js
rename to src/content-linter/tests/unit/frontmatter-video-transcripts.ts
diff --git a/src/content-linter/tests/unit/image-alt-text-exclude-start-words.js b/src/content-linter/tests/unit/image-alt-text-exclude-start-words.ts
similarity index 100%
rename from src/content-linter/tests/unit/image-alt-text-exclude-start-words.js
rename to src/content-linter/tests/unit/image-alt-text-exclude-start-words.ts
diff --git a/src/content-linter/tests/unit/internal-links-slash.js b/src/content-linter/tests/unit/internal-links-slash.ts
similarity index 100%
rename from src/content-linter/tests/unit/internal-links-slash.js
rename to src/content-linter/tests/unit/internal-links-slash.ts
diff --git a/src/content-linter/tests/unit/link-punctuation.js b/src/content-linter/tests/unit/link-punctuation.ts
similarity index 100%
rename from src/content-linter/tests/unit/link-punctuation.js
rename to src/content-linter/tests/unit/link-punctuation.ts
diff --git a/src/content-linter/tests/unit/rule-filtering.js b/src/content-linter/tests/unit/rule-filtering.ts
similarity index 100%
rename from src/content-linter/tests/unit/rule-filtering.js
rename to src/content-linter/tests/unit/rule-filtering.ts
diff --git a/src/content-linter/tests/unit/table-column-integrity-simple.js b/src/content-linter/tests/unit/table-column-integrity-simple.ts
similarity index 89%
rename from src/content-linter/tests/unit/table-column-integrity-simple.js
rename to src/content-linter/tests/unit/table-column-integrity-simple.ts
index 6d45e01a3f07..de8eb55724a9 100644
--- a/src/content-linter/tests/unit/table-column-integrity-simple.js
+++ b/src/content-linter/tests/unit/table-column-integrity-simple.ts
@@ -22,8 +22,8 @@ describe(tableColumnIntegrity.names.join(' - '), () => {
     const errors = result.markdown
     expect(errors.length).toBe(1)
     expect(errors[0].lineNumber).toBe(3)
-    if (errors[0].detail) {
-      expect(errors[0].detail).toContain('Table row has 3 columns but header has 2')
+    if ((errors[0] as any).detail) {
+      expect((errors[0] as any).detail).toContain('Table row has 3 columns but header has 2')
     } else if (errors[0].errorDetail) {
       expect(errors[0].errorDetail).toContain('Table row has 3 columns but header has 2')
     } else {
@@ -38,8 +38,8 @@ describe(tableColumnIntegrity.names.join(' - '), () => {
     const errors = result.markdown
     expect(errors.length).toBe(1)
     expect(errors[0].lineNumber).toBe(3)
-    if (errors[0].detail) {
-      expect(errors[0].detail).toContain('Table row has 2 columns but header has 3')
+    if ((errors[0] as any).detail) {
+      expect((errors[0] as any).detail).toContain('Table row has 2 columns but header has 3')
     } else if (errors[0].errorDetail) {
       expect(errors[0].errorDetail).toContain('Table row has 2 columns but header has 3')
     } else {
diff --git a/src/content-render/tests/data.js b/src/content-render/tests/data.ts
similarity index 89%
rename from src/content-render/tests/data.js
rename to src/content-render/tests/data.ts
index 6cc104d4d2e8..322509b2ba63 100644
--- a/src/content-render/tests/data.js
+++ b/src/content-render/tests/data.ts
@@ -6,7 +6,8 @@ import nonEnterpriseDefaultVersion from '@/versions/lib/non-enterprise-default-v
 import { DataDirectory } from '@/tests/helpers/data-directory'
 
 describe('data tag', () => {
-  let dd
+  // Using 'any' type as DataDirectory is from data-directory.js which lacks type definitions
+  let dd: any
   const enDirBefore = languages.en.dir
 
   beforeAll(() => {
@@ -41,7 +42,7 @@ describe('data tag', () => {
       currentLanguage: 'en',
       currentPath: '/en/liquid-tags/good-data-variable',
     }
-    const rendered = await page.render(context)
+    const rendered = await page!.render(context)
     // The test fixture contains:
     //   {% data variables.stuff.foo %}
     // which we control the value of here in the test.
@@ -57,7 +58,7 @@ describe('data tag', () => {
       currentPath: '/en/liquid-tags/bad-data-variable',
       currentLanguage: 'en',
     }
-    await expect(page.render(context)).rejects.toThrow(
+    await expect(page!.render(context)).rejects.toThrow(
       "Can't find the key 'foo.bar.tipu' in the scope., line:2, col:1",
     )
   })
diff --git a/src/content-render/tests/liquid-helpers.js b/src/content-render/tests/liquid-helpers.ts
similarity index 90%
rename from src/content-render/tests/liquid-helpers.js
rename to src/content-render/tests/liquid-helpers.ts
index ee4bdce298a6..34042034ecaf 100644
--- a/src/content-render/tests/liquid-helpers.js
+++ b/src/content-render/tests/liquid-helpers.ts
@@ -7,8 +7,10 @@ import { DataDirectory } from '@/tests/helpers/data-directory'
 describe('liquid helper tags', () => {
   vi.setConfig({ testTimeout: 60 * 1000 })
 
-  const context = {}
-  let dd
+  // Using 'any' type as context is a test fixture with dynamic properties set in beforeAll
+  const context: any = {}
+  // Using 'any' type as DataDirectory is from data-directory.js which lacks type definitions
+  let dd: any
   const enDirBefore = languages.en.dir
 
   beforeAll(() => {
diff --git a/src/content-render/tests/octicon.js b/src/content-render/tests/octicon.ts
similarity index 100%
rename from src/content-render/tests/octicon.js
rename to src/content-render/tests/octicon.ts
diff --git a/src/data-directory/tests/data-schemas.js b/src/data-directory/tests/data-schemas.ts
similarity index 82%
rename from src/data-directory/tests/data-schemas.js
rename to src/data-directory/tests/data-schemas.ts
index e7a3ae2bb00a..d2966023a63e 100644
--- a/src/data-directory/tests/data-schemas.js
+++ b/src/data-directory/tests/data-schemas.ts
@@ -4,6 +4,7 @@ import { extname, basename } from 'path'
 
 import walk from 'walk-sync'
 import { beforeAll, describe, expect, test } from 'vitest'
+import type { ValidateFunction, SchemaObject } from 'ajv'
 
 import { getJsonValidator, validateJson } from '@/tests/lib/validate-json-schema'
 import { formatAjvErrors } from '@/tests/helpers/schemas'
@@ -19,7 +20,8 @@ const yamlWalkOptions = {
 }
 
 for (const dataDir of directorySchemas) {
-  let schema, validate
+  let schema: SchemaObject
+  let validate: ValidateFunction
   const dataDirectoryName = basename(dataDir)
   const yamlFileList = walk(dataDir, yamlWalkOptions).sort()
 
@@ -32,7 +34,7 @@ for (const dataDir of directorySchemas) {
     test.each(yamlFileList)('%p', async (yamlAbsPath) => {
       const yamlContent = yaml.load(readFileSync(yamlAbsPath, 'utf8'))
       const isValid = validate(yamlContent)
-      const formattedErrors = isValid ? validate.errors : formatAjvErrors(validate.errors)
+      const formattedErrors = isValid ? undefined : formatAjvErrors(validate.errors || [])
       expect(isValid, formattedErrors).toBe(true)
     })
   })
@@ -43,6 +45,7 @@ describe('single data files', () => {
     const ymlData = yaml.load(readFileSync(filepath, 'utf8'))
     const schema = (await import(dataSchemas[filepath])).default
     const { isValid, errors } = validateJson(schema, ymlData)
-    expect(isValid, errors).toBe(true)
+    const formattedErrors = isValid ? undefined : formatAjvErrors(errors || [])
+    expect(isValid, formattedErrors).toBe(true)
   })
 })
diff --git a/src/data-directory/tests/index.js b/src/data-directory/tests/index.ts
similarity index 93%
rename from src/data-directory/tests/index.js
rename to src/data-directory/tests/index.ts
index f33ca9b5cbfd..6ba9dde7b2a1 100644
--- a/src/data-directory/tests/index.js
+++ b/src/data-directory/tests/index.ts
@@ -20,7 +20,7 @@ describe('data-directory', () => {
   })
 
   test('option: preprocess function', async () => {
-    const preprocess = function (content) {
+    const preprocess = function (content: string) {
       return content.replace('markdown', 'MARKDOWN')
     }
     const data = dataDirectory(fixturesDir, { preprocess })
@@ -35,7 +35,7 @@ describe('data-directory', () => {
   })
 
   test('option: ignorePatterns', async () => {
-    const ignorePatterns = []
+    const ignorePatterns: RegExp[] = []
 
     // README is ignored by default
     expect('README' in dataDirectory(fixturesDir)).toBe(false)
diff --git a/src/frame/middleware/set-fastly-surrogate-key.js b/src/frame/middleware/set-fastly-surrogate-key.ts
similarity index 60%
rename from src/frame/middleware/set-fastly-surrogate-key.js
rename to src/frame/middleware/set-fastly-surrogate-key.ts
index fd72df15abb9..1fa0befa5b94 100644
--- a/src/frame/middleware/set-fastly-surrogate-key.js
+++ b/src/frame/middleware/set-fastly-surrogate-key.ts
@@ -1,3 +1,5 @@
+import type { NextFunction } from 'express'
+
 // Fastly provides a Soft Purge feature that allows you to mark content as outdated (stale) instead of permanently
 // purging and thereby deleting it from Fastly's caches. Objects invalidated with Soft Purge will be treated as
 // outdated (stale) while Fastly fetches a new version from origin.
@@ -14,7 +16,8 @@ export const SURROGATE_ENUMS = {
   MANUAL: 'manual-purge',
 }
 
-export function setFastlySurrogateKey(res, enumKey, isCustomKey = false) {
+// Using 'any' type for res parameter to maintain compatibility with Express Response objects
+export function setFastlySurrogateKey(res: any, enumKey: string, isCustomKey = false) {
   if (process.env.NODE_ENV !== 'production') {
     if (!isCustomKey && !Object.values(SURROGATE_ENUMS).includes(enumKey)) {
       throw new Error(
@@ -27,17 +30,21 @@ export function setFastlySurrogateKey(res, enumKey, isCustomKey = false) {
   res.set(KEY, enumKey)
 }
 
-export function setDefaultFastlySurrogateKey(req, res, next) {
+// Using 'any' type for req and res parameters to maintain backward compatibility with test mock objects
+// that don't fully implement ExtendedRequest and Response interfaces
+export function setDefaultFastlySurrogateKey(req: any, res: any, next: NextFunction) {
   res.set(KEY, `${SURROGATE_ENUMS.DEFAULT} ${makeLanguageSurrogateKey()}`)
   return next()
 }
 
-export function setLanguageFastlySurrogateKey(req, res, next) {
+// Using 'any' type for req and res parameters to maintain backward compatibility with test mock objects
+// that don't fully implement ExtendedRequest and Response interfaces
+export function setLanguageFastlySurrogateKey(req: any, res: any, next: NextFunction) {
   res.set(KEY, `${SURROGATE_ENUMS.DEFAULT} ${makeLanguageSurrogateKey(req.language)}`)
   return next()
 }
 
-export function makeLanguageSurrogateKey(langCode) {
+export function makeLanguageSurrogateKey(langCode?: string) {
   if (!langCode) {
     return 'no-language'
   }
diff --git a/src/frame/tests/secure-files.js b/src/frame/tests/secure-files.ts
similarity index 100%
rename from src/frame/tests/secure-files.js
rename to src/frame/tests/secure-files.ts
diff --git a/src/frame/tests/toc-links.js b/src/frame/tests/toc-links.ts
similarity index 86%
rename from src/frame/tests/toc-links.js
rename to src/frame/tests/toc-links.ts
index 73a7f1776a32..3288fb21799b 100644
--- a/src/frame/tests/toc-links.js
+++ b/src/frame/tests/toc-links.ts
@@ -3,6 +3,7 @@ import { describe, expect, test, vi } from 'vitest'
 import { loadPageMap, loadPages } from '@/frame/lib/page-data'
 import { renderContent } from '@/content-render/index'
 import { allVersions } from '@/versions/lib/all-versions'
+import type { Permalink } from '@/types'
 
 describe('toc links', () => {
   vi.setConfig({ testTimeout: 3 * 60 * 1000 })
@@ -20,7 +21,8 @@ describe('toc links', () => {
     for (const pageVersion of Object.keys(allVersions)) {
       for (const page of englishIndexPages) {
         // skip page if it doesn't have a permalink for the current product version
-        if (!page.permalinks.some((permalink) => permalink.pageVersion === pageVersion)) continue
+        if (!page.permalinks.some((permalink: Permalink) => permalink.pageVersion === pageVersion))
+          continue
 
         // build fake context object for rendering the page
         const context = {
@@ -38,7 +40,7 @@ describe('toc links', () => {
         } catch (err) {
           issues.push({
             'TOC path': page.relativePath,
-            error: err.message,
+            error: err instanceof Error ? err.message : String(err),
             pageVersion,
           })
         }
diff --git a/src/graphql/tests/get-schema-files.js b/src/graphql/tests/get-schema-files.ts
similarity index 85%
rename from src/graphql/tests/get-schema-files.js
rename to src/graphql/tests/get-schema-files.ts
index d3479a5b1dc5..bf228be99a50 100644
--- a/src/graphql/tests/get-schema-files.js
+++ b/src/graphql/tests/get-schema-files.ts
@@ -10,10 +10,15 @@ import {
   getPreviews,
 } from '../lib/index'
 
+interface GraphqlType {
+  kind: string
+  type: string
+}
+
 describe('graphql schema', () => {
-  const graphqlTypes = JSON.parse(readFileSync('src/graphql/lib/types.json')).map(
-    (item) => item.kind,
-  )
+  const graphqlTypes = (
+    JSON.parse(readFileSync('src/graphql/lib/types.json', 'utf-8')) as GraphqlType[]
+  ).map((item) => item.kind)
   for (const version in allVersions) {
     for (const type of graphqlTypes) {
       test(`getting the GraphQL ${type} schema works for ${version}`, async () => {
diff --git a/src/graphql/tests/server-rendering.js b/src/graphql/tests/server-rendering.ts
similarity index 91%
rename from src/graphql/tests/server-rendering.js
rename to src/graphql/tests/server-rendering.ts
index f79e388cab31..9219b28ce902 100644
--- a/src/graphql/tests/server-rendering.js
+++ b/src/graphql/tests/server-rendering.ts
@@ -2,6 +2,7 @@ import { describe, expect, test } from 'vitest'
 
 import { getDOM } from '@/tests/helpers/e2etest'
 import { loadPages } from '@/frame/lib/page-data'
+import type { Permalink } from '@/types'
 
 const pageList = await loadPages(undefined, ['en'])
 
@@ -27,7 +28,9 @@ describe('server rendering certain GraphQL pages', () => {
   )
   const nonFPTPermalinks = autogeneratedPages
     .map((page) =>
-      page.permalinks.find((permalink) => permalink.pageVersion !== 'free-pro-team@latest'),
+      page.permalinks.find(
+        (permalink: Permalink) => permalink.pageVersion !== 'free-pro-team@latest',
+      ),
     )
     .filter(Boolean)
   const nonFPTPermalinksHrefs = nonFPTPermalinks.map((permalink) => {
diff --git a/src/tests/helpers/check-url.js b/src/tests/helpers/check-url.ts
similarity index 84%
rename from src/tests/helpers/check-url.js
rename to src/tests/helpers/check-url.ts
index ef8766094760..ad878eb1657b 100644
--- a/src/tests/helpers/check-url.js
+++ b/src/tests/helpers/check-url.ts
@@ -13,7 +13,7 @@ const liquidEndRex = /{%-?\s*endif\s*-?%}$/
 //   {% ifversion ghes%}/foo/bar{%endif %}
 //
 // And if no liquid, just return as is.
-function stripLiquid(text) {
+function stripLiquid(text: string): string {
   if (liquidStartRex.test(text) && liquidEndRex.test(text)) {
     return text.replace(liquidStartRex, '').replace(liquidEndRex, '').trim()
   } else if (text.includes('{')) {
@@ -26,7 +26,9 @@ function stripLiquid(text) {
 // return undefined if it can found as a known page.
 // Otherwise, return an object with information that is used to
 // print a useful test error message in the assertion.
-export function checkURL(uri, index, redirectsContext) {
+// Using 'any' type for redirectsContext parameter as it's a complex context object
+// with dynamic structure that would require extensive type definitions
+export function checkURL(uri: string, index: number, redirectsContext: any) {
   const url = `/en${stripLiquid(uri).split('#')[0]}`
   if (!(url in redirectsContext.pages)) {
     // Some are written without a version, but don't work with the
diff --git a/src/tests/helpers/schemas/products-schema.js b/src/tests/helpers/schemas/products-schema.ts
similarity index 100%
rename from src/tests/helpers/schemas/products-schema.js
rename to src/tests/helpers/schemas/products-schema.ts
diff --git a/src/versions/lib/all-versions.d.ts b/src/versions/lib/all-versions.d.ts
deleted file mode 100644
index ea2ad0058c78..000000000000
--- a/src/versions/lib/all-versions.d.ts
+++ /dev/null
@@ -1,13 +0,0 @@
-import type { AllVersions } from '@/types'
-
-export const allVersionKeys: string[]
-
-export const allVersionShortnames: Record<string, string>
-
-export declare function isApiVersioned(version: string): boolean
-
-export declare function getDocsVersion(openApiVersion: string): string
-
-export declare function getOpenApiVersion(version: string): string
-
-export const allVersions: AllVersions
diff --git a/src/versions/tests/versions.js b/src/versions/tests/versions.ts
similarity index 80%
rename from src/versions/tests/versions.js
rename to src/versions/tests/versions.ts
index 51fac5285997..3bf575cc3334 100644
--- a/src/versions/tests/versions.js
+++ b/src/versions/tests/versions.ts
@@ -6,6 +6,7 @@ import { latest } from '@/versions/lib/enterprise-server-releases'
 import schema from '@/tests/helpers/schemas/versions-schema'
 import nonEnterpriseDefaultVersion from '@/versions/lib/non-enterprise-default-version'
 import { formatAjvErrors } from '@/tests/helpers/schemas'
+import type { Version } from '@/types'
 
 const validate = getJsonValidator(schema)
 
@@ -16,12 +17,13 @@ describe('versions module', () => {
   })
 
   test('every version is valid', () => {
-    Object.values(allVersions).forEach((versionObj) => {
+    Object.values(allVersions).forEach((versionObj: Version) => {
+      const versionName = versionObj.version
       const isValid = validate(versionObj)
-      let errors
+      let errors: string | undefined
 
       if (!isValid) {
-        errors = `version '${versionObj.version}': ${formatAjvErrors(validate.errors)}`
+        errors = `version '${versionName}': ${formatAjvErrors(validate.errors || [])}`
       }
 
       expect(isValid, errors).toBe(true)
@@ -29,7 +31,7 @@ describe('versions module', () => {
   })
 
   test('check REST api calendar date versioned versions set to correct latestApiVersion', () => {
-    Object.values(allVersions).forEach((versionObj) => {
+    Object.values(allVersions).forEach((versionObj: Version) => {
       if (versionObj.apiVersions.length > 0) {
         const latestApiVersion = versionObj.latestApiVersion
         const apiVersions = versionObj.apiVersions