diff --git a/src/content-linter/lib/default-markdownlint-options.js b/src/content-linter/lib/default-markdownlint-options.js
deleted file mode 100644
index ccdda0329a..0000000000
--- a/src/content-linter/lib/default-markdownlint-options.js
+++ /dev/null
@@ -1,3 +0,0 @@
-export const defaultConfig = {
-  default: false,
-}
diff --git a/src/content-linter/lib/default-markdownlint-options.ts b/src/content-linter/lib/default-markdownlint-options.ts
new file mode 100644
index 0000000000..d21f286179
--- /dev/null
+++ b/src/content-linter/lib/default-markdownlint-options.ts
@@ -0,0 +1,5 @@
+import type { Configuration } from 'markdownlint'
+
+export const defaultConfig: Configuration = {
+  default: false,
+}
diff --git a/src/content-linter/lib/helpers/get-rules.d.ts b/src/content-linter/lib/helpers/get-rules.d.ts
deleted file mode 100644
index 694e3ec214..0000000000
--- a/src/content-linter/lib/helpers/get-rules.d.ts
+++ /dev/null
@@ -1,8 +0,0 @@
-// get-rules.d.ts
-// This is a declaration file for get-rules.js
-
-import type { Rule, RuleConfig } from '../../types'
-
-export const customRules: Rule[]
-export const allRules: Rule[]
-export const allConfig: RuleConfig
diff --git a/src/content-linter/lib/helpers/get-rules.js b/src/content-linter/lib/helpers/get-rules.js
deleted file mode 100644
index efa98024f5..0000000000
--- a/src/content-linter/lib/helpers/get-rules.js
+++ /dev/null
@@ -1,8 +0,0 @@
-import rules from '../../../../node_modules/markdownlint/lib/rules'
-import { gitHubDocsMarkdownlint } from '../linting-rules/index'
-import { baseConfig } from '../../style/base'
-import { customConfig } from '../../style/github-docs'
-
-export const customRules = gitHubDocsMarkdownlint.rules
-export const allRules = [...rules, ...gitHubDocsMarkdownlint.rules]
-export const allConfig = { ...baseConfig, ...customConfig }
diff --git a/src/content-linter/lib/helpers/get-rules.ts b/src/content-linter/lib/helpers/get-rules.ts
new file mode 100644
index 0000000000..eaeaf8660a
--- /dev/null
+++ b/src/content-linter/lib/helpers/get-rules.ts
@@ -0,0 +1,12 @@
+import { gitHubDocsMarkdownlint } from '@/content-linter/lib/linting-rules/index'
+import { baseConfig } from '@/content-linter/style/base'
+import { customConfig } from '@/content-linter/style/github-docs'
+import type { Rule } from '@/content-linter/types'
+
+// Import markdownlint rules - external library without TypeScript declarations
+// @ts-ignore - markdownlint doesn't provide TypeScript declarations
+import markdownlintRules from '../../../../node_modules/markdownlint/lib/rules'
+
+export const customRules: Rule[] = gitHubDocsMarkdownlint.rules
+export const allRules: any[] = [...markdownlintRules, ...gitHubDocsMarkdownlint.rules]
+export const allConfig: Record<string, any> = { ...baseConfig, ...customConfig }
diff --git a/src/content-linter/lib/helpers/unified-formatter-options.js b/src/content-linter/lib/helpers/unified-formatter-options.js
deleted file mode 100644
index d5efae1751..0000000000
--- a/src/content-linter/lib/helpers/unified-formatter-options.js
+++ /dev/null
@@ -1,9 +0,0 @@
-export const MARKDOWN_OPTIONS = {
-  bullet: '*',
-  emphasis: '_',
-  closeAtx: false,
-  fence: '`',
-  fences: true,
-  incrementListMarker: false,
-  strong: '*',
-}
diff --git a/src/content-linter/lib/helpers/unified-formatter-options.ts b/src/content-linter/lib/helpers/unified-formatter-options.ts
new file mode 100644
index 0000000000..a4d20e8daa
--- /dev/null
+++ b/src/content-linter/lib/helpers/unified-formatter-options.ts
@@ -0,0 +1,19 @@
+interface MarkdownFormatterOptions {
+  bullet: string
+  emphasis: string
+  closeAtx: boolean
+  fence: string
+  fences: boolean
+  incrementListMarker: boolean
+  strong: string
+}
+
+export const MARKDOWN_OPTIONS: MarkdownFormatterOptions = {
+  bullet: '*',
+  emphasis: '_',
+  closeAtx: false,
+  fence: '`',
+  fences: true,
+  incrementListMarker: false,
+  strong: '*',
+}
diff --git a/src/content-render/liquid/error-handling.js b/src/content-render/liquid/error-handling.ts
similarity index 70%
rename from src/content-render/liquid/error-handling.js
rename to src/content-render/liquid/error-handling.ts
index d131cb7dd5..c37e1a4b2e 100644
--- a/src/content-render/liquid/error-handling.js
+++ b/src/content-render/liquid/error-handling.ts
@@ -1,9 +1,9 @@
 // If 'THROW_ON_EMPTY' is set and it's value is '0' or 'false' it becomes
 // false. Or true if it's 'true' or '1'.
-export const THROW_ON_EMPTY = Boolean(
+export const THROW_ON_EMPTY: boolean = Boolean(
   process.env.THROW_ON_EMPTY
     ? JSON.parse(process.env.THROW_ON_EMPTY)
-    : JSON.parse(process.env.CI || process.env.NODE_ENV !== 'production'),
+    : JSON.parse(String(process.env.CI || process.env.NODE_ENV !== 'production')),
 )
 
 export class DataReferenceError extends Error {}
diff --git a/src/content-render/liquid/ifversion-supported-operators.js b/src/content-render/liquid/ifversion-supported-operators.js
deleted file mode 100644
index 03234759e5..0000000000
--- a/src/content-render/liquid/ifversion-supported-operators.js
+++ /dev/null
@@ -1 +0,0 @@
-export default ['=', '<', '>', '!=']
diff --git a/src/content-render/liquid/ifversion-supported-operators.ts b/src/content-render/liquid/ifversion-supported-operators.ts
new file mode 100644
index 0000000000..735c32f40c
--- /dev/null
+++ b/src/content-render/liquid/ifversion-supported-operators.ts
@@ -0,0 +1,5 @@
+const ifversionSupportedOperators = ['=', '<', '>', '!='] as const
+
+export type IfversionSupportedOperator = (typeof ifversionSupportedOperators)[number]
+
+export default ifversionSupportedOperators
diff --git a/src/content-render/liquid/index.js b/src/content-render/liquid/index.ts
similarity index 69%
rename from src/content-render/liquid/index.js
rename to src/content-render/liquid/index.ts
index 412d11b75c..ec46b21a7d 100644
--- a/src/content-render/liquid/index.js
+++ b/src/content-render/liquid/index.ts
@@ -1,7 +1,7 @@
 import { processLiquidPost } from './post'
 import { engine } from './engine'
 
-export async function renderLiquid(template, context) {
+export async function renderLiquid(template: string, context: any): Promise<string> {
   template = await engine.parseAndRender(template, context)
   template = processLiquidPost(template)
   return template
diff --git a/src/data-directory/lib/data-schemas/code-languages.js b/src/data-directory/lib/data-schemas/code-languages.js
deleted file mode 100644
index e3c89a434f..0000000000
--- a/src/data-directory/lib/data-schemas/code-languages.js
+++ /dev/null
@@ -1,16 +0,0 @@
-export default {
-  type: 'object',
-  additionalProperties: {
-    type: 'object',
-    required: ['name', 'comment'],
-    properties: {
-      name: {
-        type: 'string',
-      },
-      comment: {
-        type: 'string',
-        enum: ['number', 'slash', 'percent', 'hyphen', 'xml', 'none'],
-      },
-    },
-  },
-}
diff --git a/src/data-directory/lib/data-schemas/code-languages.ts b/src/data-directory/lib/data-schemas/code-languages.ts
new file mode 100644
index 0000000000..aa7f48e996
--- /dev/null
+++ b/src/data-directory/lib/data-schemas/code-languages.ts
@@ -0,0 +1,37 @@
+interface CodeLanguageProperties {
+  name: {
+    type: 'string'
+  }
+  comment: {
+    type: 'string'
+    enum: ['number', 'slash', 'percent', 'hyphen', 'xml', 'none']
+  }
+}
+
+interface CodeLanguageSchema {
+  type: 'object'
+  additionalProperties: {
+    type: 'object'
+    required: ['name', 'comment']
+    properties: CodeLanguageProperties
+  }
+}
+
+const codeLanguagesSchema: CodeLanguageSchema = {
+  type: 'object',
+  additionalProperties: {
+    type: 'object',
+    required: ['name', 'comment'],
+    properties: {
+      name: {
+        type: 'string',
+      },
+      comment: {
+        type: 'string',
+        enum: ['number', 'slash', 'percent', 'hyphen', 'xml', 'none'],
+      },
+    },
+  },
+}
+
+export default codeLanguagesSchema
diff --git a/src/data-directory/lib/data-schemas/index.ts b/src/data-directory/lib/data-schemas/index.ts
index e8d58ceb5e..160483e39d 100644
--- a/src/data-directory/lib/data-schemas/index.ts
+++ b/src/data-directory/lib/data-schemas/index.ts
@@ -4,10 +4,10 @@ interface DataSchemas {
 
 const dataSchemas: DataSchemas = {
   'data/features': '@/data-directory/lib/data-schemas/features.js',
-  'data/variables': '@/data-directory/lib/data-schemas/variables.js',
+  'data/variables': '@/data-directory/lib/data-schemas/variables',
   'data/learning-tracks': '@/data-directory/lib/data-schemas/learning-tracks.js',
   'data/release-notes': '@/data-directory/lib/data-schemas/release-notes.js',
-  'data/code-languages.yml': '@/data-directory/lib/data-schemas/code-languages.js',
+  'data/code-languages.yml': '@/data-directory/lib/data-schemas/code-languages',
   'data/glossaries/candidates.yml': '@/data-directory/lib/data-schemas/glossaries-candidates.js',
   'data/glossaries/external.yml': '@/data-directory/lib/data-schemas/glossaries-external.js',
 }
diff --git a/src/data-directory/lib/data-schemas/variables.js b/src/data-directory/lib/data-schemas/variables.js
deleted file mode 100644
index 3cec2cc6ff..0000000000
--- a/src/data-directory/lib/data-schemas/variables.js
+++ /dev/null
@@ -1,7 +0,0 @@
-export default {
-  type: 'object',
-  additionalProperties: {
-    type: 'string',
-    lintable: true,
-  },
-}
diff --git a/src/data-directory/lib/data-schemas/variables.ts b/src/data-directory/lib/data-schemas/variables.ts
new file mode 100644
index 0000000000..62ba9e1937
--- /dev/null
+++ b/src/data-directory/lib/data-schemas/variables.ts
@@ -0,0 +1,17 @@
+interface VariableSchema {
+  type: 'object'
+  additionalProperties: {
+    type: 'string'
+    lintable: boolean
+  }
+}
+
+const variablesSchema: VariableSchema = {
+  type: 'object',
+  additionalProperties: {
+    type: 'string',
+    lintable: true,
+  },
+}
+
+export default variablesSchema
diff --git a/src/frame/lib/encode-bracketed-parentheses.js b/src/frame/lib/encode-bracketed-parentheses.ts
similarity index 69%
rename from src/frame/lib/encode-bracketed-parentheses.js
rename to src/frame/lib/encode-bracketed-parentheses.ts
index e3e4b30403..d0df9a4a12 100644
--- a/src/frame/lib/encode-bracketed-parentheses.js
+++ b/src/frame/lib/encode-bracketed-parentheses.ts
@@ -1,6 +1,6 @@
 // prevent `[foo] (bar)` strings with a space between from being interpreted as markdown links
 // by encoding the space character
 
-export default function encodeBracketedParentheses(input) {
+export default function encodeBracketedParentheses(input: string): string {
   return input.replace(/] \(/g, ']&nbsp;(')
 }
diff --git a/src/frame/lib/read-file-contents.js b/src/frame/lib/read-file-contents.ts
similarity index 55%
rename from src/frame/lib/read-file-contents.js
rename to src/frame/lib/read-file-contents.ts
index 9b3aefc698..299efed569 100644
--- a/src/frame/lib/read-file-contents.js
+++ b/src/frame/lib/read-file-contents.ts
@@ -6,8 +6,9 @@ import fm from './frontmatter'
 /**
  * Read only the frontmatter from  file
  */
-export default async function fmfromf(filepath) {
-  let fileContent = await fs.readFile(filepath, 'utf8')
+// Using any type for return value because frontmatter structure is complex and varies
+export default async function fmfromf(filepath: string): Promise<any> {
+  let fileContent: string = await fs.readFile(filepath, 'utf8')
 
   fileContent = encodeBracketedParentheses(fileContent)
 
diff --git a/src/frame/tests/gitignore.js b/src/frame/tests/gitignore.ts
similarity index 57%
rename from src/frame/tests/gitignore.js
rename to src/frame/tests/gitignore.ts
index 989f42e63a..760ac36341 100644
--- a/src/frame/tests/gitignore.js
+++ b/src/frame/tests/gitignore.ts
@@ -3,9 +3,9 @@ import path from 'path'
 
 import { describe, expect, test } from 'vitest'
 
-const gitignorePath = path.join(process.cwd(), '.gitignore')
-const gitignore = await fs.readFile(gitignorePath, 'utf8')
-const entries = gitignore.split(/\r?\n/)
+const gitignorePath: string = path.join(process.cwd(), '.gitignore')
+const gitignore: string = await fs.readFile(gitignorePath, 'utf8')
+const entries: string[] = gitignore.split(/\r?\n/)
 
 describe('.gitignore file', () => {
   test('includes an entry for .env', () => {
diff --git a/src/rest/pages/category.tsx b/src/rest/pages/category.tsx
index e14eaf70fc..0a7f1d5701 100644
--- a/src/rest/pages/category.tsx
+++ b/src/rest/pages/category.tsx
@@ -62,7 +62,7 @@ export default function Category({
 export const getServerSideProps: GetServerSideProps<Props> = async (context) => {
   const { default: getRest, getRestMiniTocItems } = await import('@/rest/lib/index')
   const nonEnterpriseDefaultVersionModule = await import(
-    'src/versions/lib/non-enterprise-default-version.js'
+    '@/versions/lib/non-enterprise-default-version'
   )
   const nonEnterpriseDefaultVersion = nonEnterpriseDefaultVersionModule.default as string
 
diff --git a/src/tests/helpers/script-data.js b/src/tests/helpers/script-data.js
deleted file mode 100644
index b7bc3f0070..0000000000
--- a/src/tests/helpers/script-data.js
+++ /dev/null
@@ -1,13 +0,0 @@
-const NEXT_DATA_QUERY = 'script#__NEXT_DATA__'
-const PRIMER_DATA_QUERY = 'script#__PRIMER_DATA__'
-
-function getScriptData($, key) {
-  const data = $(key)
-  if (!data.length === 1) {
-    throw new Error(`Not exactly 1 element match for '${key}'. Found ${data.length}`)
-  }
-  return JSON.parse(data.get()[0].children[0].data)
-}
-
-export const getNextData = ($) => getScriptData($, NEXT_DATA_QUERY)
-export const getPrimerData = ($) => getScriptData($, PRIMER_DATA_QUERY)
diff --git a/src/tests/helpers/script-data.ts b/src/tests/helpers/script-data.ts
new file mode 100644
index 0000000000..ec028b8523
--- /dev/null
+++ b/src/tests/helpers/script-data.ts
@@ -0,0 +1,15 @@
+const NEXT_DATA_QUERY = 'script#__NEXT_DATA__'
+const PRIMER_DATA_QUERY = 'script#__PRIMER_DATA__'
+
+// Using any type for $ parameter as it represents a jQuery-like selector (cheerio)
+function getScriptData($: any, key: string): any {
+  const data = $(key)
+  if (data.length !== 1) {
+    throw new Error(`Not exactly 1 element match for '${key}'. Found ${data.length}`)
+  }
+  return JSON.parse(data.get()[0].children[0].data)
+}
+
+// Using any types for cheerio/jQuery-like objects and parsed JSON data
+export const getNextData = ($: any): any => getScriptData($, NEXT_DATA_QUERY)
+export const getPrimerData = ($: any): any => getScriptData($, PRIMER_DATA_QUERY)
diff --git a/src/types/markdownlint-rules.d.ts b/src/types/markdownlint-rules.d.ts
new file mode 100644
index 0000000000..7e7204208e
--- /dev/null
+++ b/src/types/markdownlint-rules.d.ts
@@ -0,0 +1,7 @@
+// Simple type declaration for markdownlint external library
+// Using any types to avoid complex typing for external dependencies
+
+declare module '../../../../node_modules/markdownlint/lib/rules' {
+  const rules: any[]
+  export = rules
+}
diff --git a/src/versions/lib/non-enterprise-default-version.js b/src/versions/lib/non-enterprise-default-version.ts
similarity index 94%
rename from src/versions/lib/non-enterprise-default-version.js
rename to src/versions/lib/non-enterprise-default-version.ts
index 5a438f0bf6..23edb11252 100644
--- a/src/versions/lib/non-enterprise-default-version.js
+++ b/src/versions/lib/non-enterprise-default-version.ts
@@ -1,6 +1,7 @@
 import { allVersions } from '@/versions/lib/all-versions'
+
 const nonEnterpriseDefaultVersion = Object.values(allVersions).find(
   (version) => version.nonEnterpriseDefault,
-).version
+)!.version
 
 export default nonEnterpriseDefaultVersion
diff --git a/src/versions/lib/remove-fpt-from-path.js b/src/versions/lib/remove-fpt-from-path.ts
similarity index 86%
rename from src/versions/lib/remove-fpt-from-path.js
rename to src/versions/lib/remove-fpt-from-path.ts
index d93a9a29a1..32ee9ded6f 100644
--- a/src/versions/lib/remove-fpt-from-path.js
+++ b/src/versions/lib/remove-fpt-from-path.ts
@@ -4,6 +4,6 @@ import nonEnterpriseDefaultVersion from './non-enterprise-default-version'
 // This is a convenience function to remove free-pro-team@latest from all
 // **user-facing** aspects of the site (particularly URLs) while continuing to support
 // free-pro-team@latest as a version both in the codebase and in content/data files.
-export default function removeFPTFromPath(path) {
+export default function removeFPTFromPath(path: string): string {
   return slash(path.replace(`/${nonEnterpriseDefaultVersion}`, ''))
 }