diff --git a/packages/url/README.md b/packages/url/README.md
index ae5b6bf945fe5f..86159b22c20343 100644
--- a/packages/url/README.md
+++ b/packages/url/README.md
@@ -28,8 +28,8 @@ const newURL = addQueryArgs( 'https://google.com', { q: 'test' } ); // https://g
 
 _Parameters_
 
--   _url_ `[string]`: URL to which arguments should be appended. If omitted, only the resulting querystring is returned.
--   _args_ `[Object]`: Query arguments to apply to URL.
+-   _url_ `string`: URL to which arguments should be appended. If omitted, only the resulting querystring is returned.
+-   _args_ `Record< string, unknown >`: Query arguments to apply to URL.
 
 _Returns_
 
@@ -58,7 +58,7 @@ const queryString = buildQueryString( {
 
 _Parameters_
 
--   _data_ `Record<string,*>`: Data to encode.
+-   _data_ `Record< string, unknown >`: Data to encode.
 
 _Returns_
 
@@ -99,7 +99,7 @@ const imageUrl = filterURLForDisplay(
 _Parameters_
 
 -   _url_ `string`: Original URL.
--   _maxLength_ `number|null`: URL length.
+-   _maxLength_ `number | null`: URL length.
 
 _Returns_
 
@@ -122,7 +122,7 @@ _Parameters_
 
 _Returns_
 
--   `string|void`: The authority part of the URL.
+-   `string | void`: The authority part of the URL.
 
 ### getFilename
 
@@ -141,7 +141,7 @@ _Parameters_
 
 _Returns_
 
--   `string|void`: The filename part of the URL.
+-   `string | void`: The filename part of the URL.
 
 ### getFragment
 
@@ -164,7 +164,7 @@ _Parameters_
 
 _Returns_
 
--   `string|void`: The fragment part of the URL.
+-   `string | void`: The fragment part of the URL.
 
 ### getPath
 
@@ -183,7 +183,7 @@ _Parameters_
 
 _Returns_
 
--   `string|void`: The path part of the URL.
+-   `string | void`: The path part of the URL.
 
 ### getPathAndQueryString
 
@@ -225,7 +225,7 @@ _Parameters_
 
 _Returns_
 
--   `string|void`: The protocol part of the URL.
+-   `string | void`: The protocol part of the URL.
 
 ### getQueryArg
 
@@ -244,7 +244,7 @@ _Parameters_
 
 _Returns_
 
--   `QueryArgParsed|void`: Query arg value.
+-   `QueryArgParsed | undefined`: Query arg value.
 
 ### getQueryArgs
 
@@ -283,7 +283,7 @@ _Parameters_
 
 _Returns_
 
--   `string|void`: The query string part of the URL.
+-   `string | void`: The query string part of the URL.
 
 ### hasQueryArg
 
@@ -525,7 +525,7 @@ const newUrl = removeQueryArgs(
 _Parameters_
 
 -   _url_ `string`: URL.
--   _args_ `...string`: Query Args.
+-   _args_ `string[]`: Query Args.
 
 _Returns_
 
diff --git a/packages/url/src/add-query-args.js b/packages/url/src/add-query-args.ts
similarity index 78%
rename from packages/url/src/add-query-args.js
rename to packages/url/src/add-query-args.ts
index 556a3fb706a447..1f4c05de3bbdd6 100644
--- a/packages/url/src/add-query-args.js
+++ b/packages/url/src/add-query-args.ts
@@ -10,18 +10,21 @@ import { getFragment } from './get-fragment';
  * includes query arguments, the arguments are merged with (and take precedent
  * over) the existing set.
  *
- * @param {string} [url=''] URL to which arguments should be appended. If omitted,
- *                          only the resulting querystring is returned.
- * @param {Object} [args]   Query arguments to apply to URL.
+ * @param url  URL to which arguments should be appended. If omitted,
+ *             only the resulting querystring is returned.
+ * @param args Query arguments to apply to URL.
  *
  * @example
  * ```js
  * const newURL = addQueryArgs( 'https://google.com', { q: 'test' } ); // https://google.com/?q=test
  * ```
  *
- * @return {string} URL with arguments applied.
+ * @return URL with arguments applied.
  */
-export function addQueryArgs( url = '', args ) {
+export function addQueryArgs(
+	url: string = '',
+	args?: Record< string, unknown >
+): string {
 	// If no arguments are to be appended, return original URL.
 	if ( ! args || ! Object.keys( args ).length ) {
 		return url;
diff --git a/packages/url/src/build-query-string.js b/packages/url/src/build-query-string.ts
similarity index 82%
rename from packages/url/src/build-query-string.js
rename to packages/url/src/build-query-string.ts
index 5f8b6a68079a5b..5315d77bae03ad 100644
--- a/packages/url/src/build-query-string.js
+++ b/packages/url/src/build-query-string.ts
@@ -18,14 +18,14 @@
  * // "simple=is%20ok&arrays%5B0%5D=are&arrays%5B1%5D=fine&arrays%5B2%5D=too&objects%5BevenNested%5D%5Bok%5D=yes"
  * ```
  *
- * @param {Record<string,*>} data Data to encode.
+ * @param data Data to encode.
  *
- * @return {string} Query string.
+ * @return Query string.
  */
-export function buildQueryString( data ) {
+export function buildQueryString( data: Record< string, unknown > ): string {
 	let string = '';
 
-	const stack = Object.entries( data );
+	const stack: Array< [ string, unknown ] > = Object.entries( data );
 
 	let pair;
 	while ( ( pair = stack.shift() ) ) {
@@ -39,7 +39,7 @@ export function buildQueryString( data ) {
 			// Push array or object values onto the stack as composed of their
 			// original key and nested index or key, retaining order by a
 			// combination of Array#reverse and Array#unshift onto the stack.
-			const valuePairs = Object.entries( value ).reverse();
+			const valuePairs = Object.entries( value as object ).reverse();
 			for ( const [ member, memberValue ] of valuePairs ) {
 				stack.unshift( [ `${ key }[${ member }]`, memberValue ] );
 			}
@@ -50,7 +50,8 @@ export function buildQueryString( data ) {
 			}
 
 			string +=
-				'&' + [ key, value ].map( encodeURIComponent ).join( '=' );
+				'&' +
+				[ key, String( value ) ].map( encodeURIComponent ).join( '=' );
 		}
 	}
 
diff --git a/packages/url/src/clean-for-slug.js b/packages/url/src/clean-for-slug.ts
similarity index 90%
rename from packages/url/src/clean-for-slug.js
rename to packages/url/src/clean-for-slug.ts
index e288f5949537ad..a6dd495ff61f4f 100644
--- a/packages/url/src/clean-for-slug.js
+++ b/packages/url/src/clean-for-slug.ts
@@ -15,11 +15,11 @@ import removeAccents from 'remove-accents';
  * except hyphens. Converts remaining string to lowercase. It does not account
  * for octets, HTML entities, or other encoded characters.
  *
- * @param {string} string Title or slug to be processed.
+ * @param string Title or slug to be processed.
  *
- * @return {string} Processed string.
+ * @return Processed string.
  */
-export function cleanForSlug( string ) {
+export function cleanForSlug( string: string ): string {
 	if ( ! string ) {
 		return '';
 	}
diff --git a/packages/url/src/filter-url-for-display.js b/packages/url/src/filter-url-for-display.ts
similarity index 88%
rename from packages/url/src/filter-url-for-display.js
rename to packages/url/src/filter-url-for-display.ts
index 436070b667a3e6..848aac53efed03 100644
--- a/packages/url/src/filter-url-for-display.js
+++ b/packages/url/src/filter-url-for-display.ts
@@ -1,8 +1,8 @@
 /**
  * Returns a URL for display.
  *
- * @param {string}      url       Original URL.
- * @param {number|null} maxLength URL length.
+ * @param url       Original URL.
+ * @param maxLength URL length.
  *
  * @example
  * ```js
@@ -10,9 +10,12 @@
  * const imageUrl = filterURLForDisplay( 'https://www.wordpress.org/wp-content/uploads/img.png', 20 ); // …ent/uploads/img.png
  * ```
  *
- * @return {string} Displayed URL.
+ * @return Displayed URL.
  */
-export function filterURLForDisplay( url, maxLength = null ) {
+export function filterURLForDisplay(
+	url: string,
+	maxLength: number | null = null
+): string {
 	if ( ! url ) {
 		return '';
 	}
diff --git a/packages/url/src/get-authority.js b/packages/url/src/get-authority.ts
similarity index 75%
rename from packages/url/src/get-authority.js
rename to packages/url/src/get-authority.ts
index f29afd5411c11d..ae11cedef0566d 100644
--- a/packages/url/src/get-authority.js
+++ b/packages/url/src/get-authority.ts
@@ -1,7 +1,7 @@
 /**
  * Returns the authority part of the URL.
  *
- * @param {string} url The full URL.
+ * @param url The full URL.
  *
  * @example
  * ```js
@@ -9,9 +9,9 @@
  * const authority2 = getAuthority( 'https://localhost:8080/test/' ); // 'localhost:8080'
  * ```
  *
- * @return {string|void} The authority part of the URL.
+ * @return The authority part of the URL.
  */
-export function getAuthority( url ) {
+export function getAuthority( url: string ): string | void {
 	const matches = /^[^\/\s:]+:(?:\/\/)?\/?([^\/\s#?]+)[\/#?]{0,1}\S*$/.exec(
 		url
 	);
diff --git a/packages/url/src/get-filename.js b/packages/url/src/get-filename.ts
similarity index 78%
rename from packages/url/src/get-filename.js
rename to packages/url/src/get-filename.ts
index d744a8da6bccfe..d5550618734370 100644
--- a/packages/url/src/get-filename.js
+++ b/packages/url/src/get-filename.ts
@@ -1,7 +1,7 @@
 /**
  * Returns the filename part of the URL.
  *
- * @param {string} url The full URL.
+ * @param url The full URL.
  *
  * @example
  * ```js
@@ -9,9 +9,9 @@
  * const filename2 = getFilename( '/this/is/a/test.png' ); // 'test.png'
  * ```
  *
- * @return {string|void} The filename part of the URL.
+ * @return The filename part of the URL.
  */
-export function getFilename( url ) {
+export function getFilename( url: string ): string | void {
 	let filename;
 
 	if ( ! url ) {
diff --git a/packages/url/src/get-fragment.js b/packages/url/src/get-fragment.ts
similarity index 75%
rename from packages/url/src/get-fragment.js
rename to packages/url/src/get-fragment.ts
index 4f15ef0ce06af8..d6a48a0bfc7cda 100644
--- a/packages/url/src/get-fragment.js
+++ b/packages/url/src/get-fragment.ts
@@ -1,7 +1,7 @@
 /**
  * Returns the fragment part of the URL.
  *
- * @param {string} url The full URL
+ * @param url The full URL
  *
  * @example
  * ```js
@@ -9,9 +9,9 @@
  * const fragment2 = getFragment( 'https://wordpress.org#another-fragment?query=true' ); // '#another-fragment'
  * ```
  *
- * @return {string|void} The fragment part of the URL.
+ * @return The fragment part of the URL.
  */
-export function getFragment( url ) {
+export function getFragment( url: string ): string | void {
 	const matches = /^\S+?(#[^\s\?]*)/.exec( url );
 	if ( matches ) {
 		return matches[ 1 ];
diff --git a/packages/url/src/get-path-and-query-string.js b/packages/url/src/get-path-and-query-string.ts
similarity index 80%
rename from packages/url/src/get-path-and-query-string.js
rename to packages/url/src/get-path-and-query-string.ts
index f858846b07802c..64869cab6272e9 100644
--- a/packages/url/src/get-path-and-query-string.js
+++ b/packages/url/src/get-path-and-query-string.ts
@@ -6,7 +6,7 @@ import { getPath, getQueryString } from '.';
 /**
  * Returns the path part and query string part of the URL.
  *
- * @param {string} url The full URL.
+ * @param url The full URL.
  *
  * @example
  * ```js
@@ -14,9 +14,9 @@ import { getPath, getQueryString } from '.';
  * const pathAndQueryString2 = getPathAndQueryString( 'https://wordpress.org/help/faq/' ); // '/help/faq'
  * ```
  *
- * @return {string} The path part and query string part of the URL.
+ * @return The path part and query string part of the URL.
  */
-export function getPathAndQueryString( url ) {
+export function getPathAndQueryString( url: string ): string {
 	const path = getPath( url );
 	const queryString = getQueryString( url );
 	let value = '/';
diff --git a/packages/url/src/get-path.js b/packages/url/src/get-path.ts
similarity index 76%
rename from packages/url/src/get-path.js
rename to packages/url/src/get-path.ts
index e9c4c717f6b92b..3a3b192d179c7c 100644
--- a/packages/url/src/get-path.js
+++ b/packages/url/src/get-path.ts
@@ -1,7 +1,7 @@
 /**
  * Returns the path part of the URL.
  *
- * @param {string} url The full URL.
+ * @param url The full URL.
  *
  * @example
  * ```js
@@ -9,9 +9,9 @@
  * const path2 = getPath( 'https://wordpress.org/help/faq/' ); // 'help/faq'
  * ```
  *
- * @return {string|void} The path part of the URL.
+ * @return The path part of the URL.
  */
-export function getPath( url ) {
+export function getPath( url: string ): string | void {
 	const matches =
 		/^[^\/\s:]+:(?:\/\/)?[^\/\s#?]+[\/]([^\s#?]+)[#?]{0,1}\S*$/.exec( url );
 	if ( matches ) {
diff --git a/packages/url/src/get-protocol.js b/packages/url/src/get-protocol.ts
similarity index 70%
rename from packages/url/src/get-protocol.js
rename to packages/url/src/get-protocol.ts
index 07b375c10f841b..f0360580c47f0e 100644
--- a/packages/url/src/get-protocol.js
+++ b/packages/url/src/get-protocol.ts
@@ -1,7 +1,7 @@
 /**
  * Returns the protocol part of the URL.
  *
- * @param {string} url The full URL.
+ * @param url The full URL.
  *
  * @example
  * ```js
@@ -9,9 +9,9 @@
  * const protocol2 = getProtocol( 'https://wordpress.org' ); // 'https:'
  * ```
  *
- * @return {string|void} The protocol part of the URL.
+ * @return  The protocol part of the URL.
  */
-export function getProtocol( url ) {
+export function getProtocol( url: string ): string | void {
 	const matches = /^([^\s:]+:)/.exec( url );
 	if ( matches ) {
 		return matches[ 1 ];
diff --git a/packages/url/src/get-query-arg.js b/packages/url/src/get-query-arg.js
deleted file mode 100644
index 05f554cd8f67d4..00000000000000
--- a/packages/url/src/get-query-arg.js
+++ /dev/null
@@ -1,29 +0,0 @@
-/**
- * Internal dependencies
- */
-import { getQueryArgs } from './get-query-args';
-
-/**
- * @typedef {{[key: string]: QueryArgParsed}} QueryArgObject
- */
-
-/**
- * @typedef {string|string[]|QueryArgObject} QueryArgParsed
- */
-
-/**
- * Returns a single query argument of the url
- *
- * @param {string} url URL.
- * @param {string} arg Query arg name.
- *
- * @example
- * ```js
- * const foo = getQueryArg( 'https://wordpress.org?foo=bar&bar=baz', 'foo' ); // bar
- * ```
- *
- * @return {QueryArgParsed|void} Query arg value.
- */
-export function getQueryArg( url, arg ) {
-	return getQueryArgs( url )[ arg ];
-}
diff --git a/packages/url/src/get-query-arg.ts b/packages/url/src/get-query-arg.ts
new file mode 100644
index 00000000000000..7bc1872a5fe429
--- /dev/null
+++ b/packages/url/src/get-query-arg.ts
@@ -0,0 +1,30 @@
+/**
+ * Internal dependencies
+ */
+import { getQueryArgs } from './get-query-args';
+
+export interface QueryArgObject {
+	[ key: string ]: QueryArgParsed;
+}
+
+export type QueryArgParsed = string | string[] | QueryArgObject;
+
+/**
+ * Returns a single query argument of the url
+ *
+ * @param url URL.
+ * @param arg Query arg name.
+ *
+ * @example
+ * ```js
+ * const foo = getQueryArg( 'https://wordpress.org?foo=bar&bar=baz', 'foo' ); // bar
+ * ```
+ *
+ * @return Query arg value.
+ */
+export function getQueryArg(
+	url: string,
+	arg: string
+): QueryArgParsed | undefined {
+	return getQueryArgs( url )[ arg ];
+}
diff --git a/packages/url/src/get-query-args.js b/packages/url/src/get-query-args.ts
similarity index 84%
rename from packages/url/src/get-query-args.js
rename to packages/url/src/get-query-args.ts
index 45626f1d8a1e94..71fe6c06c0bb45 100644
--- a/packages/url/src/get-query-args.js
+++ b/packages/url/src/get-query-args.ts
@@ -3,22 +3,19 @@
  */
 import { safeDecodeURIComponent } from './safe-decode-uri-component';
 import { getQueryString } from './get-query-string';
+import type { QueryArgParsed } from './get-query-arg';
 
-/** @typedef {import('./get-query-arg').QueryArgParsed} QueryArgParsed */
-
-/**
- * @typedef {Record<string,QueryArgParsed>} QueryArgs
- */
+type QueryArgs = Record< string, QueryArgParsed >;
 
 /**
  * Sets a value in object deeply by a given array of path segments. Mutates the
  * object reference.
  *
- * @param {Record<string,*>} object Object in which to assign.
- * @param {string[]}         path   Path segment at which to set value.
- * @param {*}                value  Value to set.
+ * @param object Object in which to assign.
+ * @param path   Path segment at which to set value.
+ * @param value  Value to set.
  */
-function setPath( object, path, value ) {
+function setPath( object: Record< string, any >, path: string[], value: any ) {
 	const length = path.length;
 	const lastIndex = length - 1;
 	for ( let i = 0; i < length; i++ ) {
@@ -61,7 +58,7 @@ function setPath( object, path, value ) {
  * Returns an object of query arguments of the given URL. If the given URL is
  * invalid or has no querystring, an empty object is returned.
  *
- * @param {string} url URL.
+ * @param url URL.
  *
  * @example
  * ```js
@@ -69,9 +66,9 @@ function setPath( object, path, value ) {
  * // { "foo": "bar", "bar": "baz" }
  * ```
  *
- * @return {QueryArgs} Query args object.
+ * @return Query args object.
  */
-export function getQueryArgs( url ) {
+export function getQueryArgs( url: string ): QueryArgs {
 	return (
 		( getQueryString( url ) || '' )
 			// Normalize space encoding, accounting for PHP URL encoding
diff --git a/packages/url/src/get-query-string.js b/packages/url/src/get-query-string.ts
similarity index 72%
rename from packages/url/src/get-query-string.js
rename to packages/url/src/get-query-string.ts
index 8624ce5c6c8b9d..91f9947a61e664 100644
--- a/packages/url/src/get-query-string.js
+++ b/packages/url/src/get-query-string.ts
@@ -1,16 +1,16 @@
 /**
  * Returns the query string part of the URL.
  *
- * @param {string} url The full URL.
+ * @param url The full URL.
  *
  * @example
  * ```js
  * const queryString = getQueryString( 'http://localhost:8080/this/is/a/test?query=true#fragment' ); // 'query=true'
  * ```
  *
- * @return {string|void} The query string part of the URL.
+ * @return The query string part of the URL.
  */
-export function getQueryString( url ) {
+export function getQueryString( url: string ): string | void {
 	let query;
 	try {
 		query = new URL( url, 'http://example.com' ).search.substring( 1 );
diff --git a/packages/url/src/has-query-arg.js b/packages/url/src/has-query-arg.ts
similarity index 64%
rename from packages/url/src/has-query-arg.js
rename to packages/url/src/has-query-arg.ts
index a940338dd16655..41cf83cc552a86 100644
--- a/packages/url/src/has-query-arg.js
+++ b/packages/url/src/has-query-arg.ts
@@ -6,16 +6,16 @@ import { getQueryArg } from './get-query-arg';
 /**
  * Determines whether the URL contains a given query arg.
  *
- * @param {string} url URL.
- * @param {string} arg Query arg name.
+ * @param url URL.
+ * @param arg Query arg name.
  *
  * @example
  * ```js
  * const hasBar = hasQueryArg( 'https://wordpress.org?foo=bar&bar=baz', 'bar' ); // true
  * ```
  *
- * @return {boolean} Whether or not the URL contains the query arg.
+ * @return Whether or not the URL contains the query arg.
  */
-export function hasQueryArg( url, arg ) {
+export function hasQueryArg( url: string, arg: string ): boolean {
 	return getQueryArg( url, arg ) !== undefined;
 }
diff --git a/packages/url/src/index.js b/packages/url/src/index.ts
similarity index 100%
rename from packages/url/src/index.js
rename to packages/url/src/index.ts
diff --git a/packages/url/src/is-email.js b/packages/url/src/is-email.ts
similarity index 66%
rename from packages/url/src/is-email.js
rename to packages/url/src/is-email.ts
index 815c4cdbd5fece..1e4ff1e1d1ae01 100644
--- a/packages/url/src/is-email.js
+++ b/packages/url/src/is-email.ts
@@ -4,15 +4,15 @@ const EMAIL_REGEXP =
 /**
  * Determines whether the given string looks like an email.
  *
- * @param {string} email The string to scrutinise.
+ * @param email The string to scrutinise.
  *
  * @example
  * ```js
  * const isEmail = isEmail( 'hello@wordpress.org' ); // true
  * ```
  *
- * @return {boolean} Whether or not it looks like an email.
+ * @return Whether or not it looks like an email.
  */
-export function isEmail( email ) {
+export function isEmail( email: string ): boolean {
 	return EMAIL_REGEXP.test( email );
 }
diff --git a/packages/url/src/is-phone-number.js b/packages/url/src/is-phone-number.ts
similarity index 68%
rename from packages/url/src/is-phone-number.js
rename to packages/url/src/is-phone-number.ts
index ed7aad1a3540ea..d482b1ad7e25b1 100644
--- a/packages/url/src/is-phone-number.js
+++ b/packages/url/src/is-phone-number.ts
@@ -3,16 +3,16 @@ const PHONE_REGEXP = /^(tel:)?(\+)?\d{6,15}$/;
 /**
  * Determines whether the given string looks like a phone number.
  *
- * @param {string} phoneNumber The string to scrutinize.
+ * @param phoneNumber The string to scrutinize.
  *
  * @example
  * ```js
  * const isPhoneNumber = isPhoneNumber('+1 (555) 123-4567'); // true
  * ```
  *
- * @return {boolean} Whether or not it looks like a phone number.
+ * @return Whether or not it looks like a phone number.
  */
-export function isPhoneNumber( phoneNumber ) {
+export function isPhoneNumber( phoneNumber: string ): boolean {
 	// Remove any separator from phone number.
 	phoneNumber = phoneNumber.replace( /[-.() ]/g, '' );
 	return PHONE_REGEXP.test( phoneNumber );
diff --git a/packages/url/src/is-url.js b/packages/url/src/is-url.ts
similarity index 77%
rename from packages/url/src/is-url.js
rename to packages/url/src/is-url.ts
index 1fda847947ed46..6cb8202aa0b7d3 100644
--- a/packages/url/src/is-url.js
+++ b/packages/url/src/is-url.ts
@@ -1,7 +1,7 @@
 /**
  * Determines whether the given string looks like a URL.
  *
- * @param {string} url The string to scrutinise.
+ * @param url The string to scrutinise.
  *
  * @example
  * ```js
@@ -11,9 +11,9 @@
  * @see https://url.spec.whatwg.org/
  * @see https://url.spec.whatwg.org/#valid-url-string
  *
- * @return {boolean} Whether or not it looks like a URL.
+ * @return Whether or not it looks like a URL.
  */
-export function isURL( url ) {
+export function isURL( url: string ): boolean {
 	// A URL can be considered value if the `URL` constructor is able to parse
 	// it. The constructor throws an error for an invalid URL.
 	try {
diff --git a/packages/url/src/is-valid-authority.js b/packages/url/src/is-valid-authority.ts
similarity index 63%
rename from packages/url/src/is-valid-authority.js
rename to packages/url/src/is-valid-authority.ts
index 4734259b34b19c..2e93840d0e530d 100644
--- a/packages/url/src/is-valid-authority.js
+++ b/packages/url/src/is-valid-authority.ts
@@ -1,7 +1,7 @@
 /**
  * Checks for invalid characters within the provided authority.
  *
- * @param {string} authority A string containing the URL authority.
+ * @param authority A string containing the URL authority.
  *
  * @example
  * ```js
@@ -9,9 +9,9 @@
  * const isNotValid = isValidAuthority( 'wordpress#org' ); // false
  * ```
  *
- * @return {boolean} True if the argument contains a valid authority.
+ * @return True if the argument contains a valid authority.
  */
-export function isValidAuthority( authority ) {
+export function isValidAuthority( authority: string ): boolean {
 	if ( ! authority ) {
 		return false;
 	}
diff --git a/packages/url/src/is-valid-fragment.js b/packages/url/src/is-valid-fragment.ts
similarity index 67%
rename from packages/url/src/is-valid-fragment.js
rename to packages/url/src/is-valid-fragment.ts
index ce19c0f859e5a7..4a6579e0c40754 100644
--- a/packages/url/src/is-valid-fragment.js
+++ b/packages/url/src/is-valid-fragment.ts
@@ -1,7 +1,7 @@
 /**
  * Checks for invalid characters within the provided fragment.
  *
- * @param {string} fragment The url fragment.
+ * @param fragment The url fragment.
  *
  * @example
  * ```js
@@ -9,9 +9,9 @@
  * const isNotValid = isValidFragment( '#invalid-#fragment' ); // false
  * ```
  *
- * @return {boolean} True if the argument contains a valid fragment.
+ * @return True if the argument contains a valid fragment.
  */
-export function isValidFragment( fragment ) {
+export function isValidFragment( fragment: string ): boolean {
 	if ( ! fragment ) {
 		return false;
 	}
diff --git a/packages/url/src/is-valid-path.js b/packages/url/src/is-valid-path.ts
similarity index 68%
rename from packages/url/src/is-valid-path.js
rename to packages/url/src/is-valid-path.ts
index 7ab5d9d95eeca7..b0f58354e7b94c 100644
--- a/packages/url/src/is-valid-path.js
+++ b/packages/url/src/is-valid-path.ts
@@ -1,7 +1,7 @@
 /**
  * Checks for invalid characters within the provided path.
  *
- * @param {string} path The URL path.
+ * @param path The URL path.
  *
  * @example
  * ```js
@@ -9,9 +9,9 @@
  * const isNotValid = isValidPath( '/invalid?test/path/' ); // false
  * ```
  *
- * @return {boolean} True if the argument contains a valid path
+ * @return True if the argument contains a valid path
  */
-export function isValidPath( path ) {
+export function isValidPath( path: string ): boolean {
 	if ( ! path ) {
 		return false;
 	}
diff --git a/packages/url/src/is-valid-protocol.js b/packages/url/src/is-valid-protocol.ts
similarity index 62%
rename from packages/url/src/is-valid-protocol.js
rename to packages/url/src/is-valid-protocol.ts
index 689768114318c4..fa8a05271a739a 100644
--- a/packages/url/src/is-valid-protocol.js
+++ b/packages/url/src/is-valid-protocol.ts
@@ -1,7 +1,7 @@
 /**
  * Tests if a url protocol is valid.
  *
- * @param {string} protocol The url protocol.
+ * @param protocol The url protocol.
  *
  * @example
  * ```js
@@ -9,9 +9,9 @@
  * const isNotValid = isValidProtocol( 'https :' ); // false
  * ```
  *
- * @return {boolean} True if the argument is a valid protocol (e.g. http:, tel:).
+ * @return True if the argument is a valid protocol (e.g. http:, tel:).
  */
-export function isValidProtocol( protocol ) {
+export function isValidProtocol( protocol: string ): boolean {
 	if ( ! protocol ) {
 		return false;
 	}
diff --git a/packages/url/src/is-valid-query-string.js b/packages/url/src/is-valid-query-string.ts
similarity index 67%
rename from packages/url/src/is-valid-query-string.js
rename to packages/url/src/is-valid-query-string.ts
index 039cede44de959..16139f52b626f5 100644
--- a/packages/url/src/is-valid-query-string.js
+++ b/packages/url/src/is-valid-query-string.ts
@@ -1,7 +1,7 @@
 /**
  * Checks for invalid characters within the provided query string.
  *
- * @param {string} queryString The query string.
+ * @param queryString The query string.
  *
  * @example
  * ```js
@@ -9,9 +9,9 @@
  * const isNotValid = isValidQueryString( 'query=true?another=false' ); // false
  * ```
  *
- * @return {boolean} True if the argument contains a valid query string.
+ * @return True if the argument contains a valid query string.
  */
-export function isValidQueryString( queryString ) {
+export function isValidQueryString( queryString: string ): boolean {
 	if ( ! queryString ) {
 		return false;
 	}
diff --git a/packages/url/src/normalize-path.js b/packages/url/src/normalize-path.ts
similarity index 89%
rename from packages/url/src/normalize-path.js
rename to packages/url/src/normalize-path.ts
index eb1cafed083657..93d7cca54f9158 100644
--- a/packages/url/src/normalize-path.js
+++ b/packages/url/src/normalize-path.ts
@@ -3,11 +3,11 @@
  * will be treated as identical, regardless of order they appear in the original
  * text.
  *
- * @param {string} path Original path.
+ * @param path Original path.
  *
- * @return {string} Normalized path.
+ * @return Normalized path.
  */
-export function normalizePath( path ) {
+export function normalizePath( path: string ): string {
 	const split = path.split( '?' );
 	const query = split[ 1 ];
 	const base = split[ 0 ];
diff --git a/packages/url/src/prepend-http.js b/packages/url/src/prepend-http.ts
similarity index 81%
rename from packages/url/src/prepend-http.js
rename to packages/url/src/prepend-http.ts
index e76488c51a4266..47ebc6e536f65d 100644
--- a/packages/url/src/prepend-http.js
+++ b/packages/url/src/prepend-http.ts
@@ -8,16 +8,16 @@ const USABLE_HREF_REGEXP = /^(?:[a-z]+:|#|\?|\.|\/)/i;
 /**
  * Prepends "http://" to a url, if it looks like something that is meant to be a TLD.
  *
- * @param {string} url The URL to test.
+ * @param url The URL to test.
  *
  * @example
  * ```js
  * const actualURL = prependHTTP( 'wordpress.org' ); // http://wordpress.org
  * ```
  *
- * @return {string} The updated URL.
+ * @return The updated URL.
  */
-export function prependHTTP( url ) {
+export function prependHTTP( url: string ): string {
 	if ( ! url ) {
 		return url;
 	}
diff --git a/packages/url/src/prepend-https.js b/packages/url/src/prepend-https.ts
similarity index 83%
rename from packages/url/src/prepend-https.js
rename to packages/url/src/prepend-https.ts
index 237bf3e18cff70..8cc062ed000eb2 100644
--- a/packages/url/src/prepend-https.js
+++ b/packages/url/src/prepend-https.ts
@@ -8,16 +8,16 @@ import { prependHTTP } from './prepend-http';
  *
  * Note: this will not replace "http://" with "https://".
  *
- * @param {string} url The URL to test.
+ * @param url The URL to test.
  *
  * @example
  * ```js
  * const actualURL = prependHTTPS( 'wordpress.org' ); // https://wordpress.org
  * ```
  *
- * @return {string} The updated URL.
+ * @return The updated URL.
  */
-export function prependHTTPS( url ) {
+export function prependHTTPS( url: string ): string {
 	if ( ! url ) {
 		return url;
 	}
diff --git a/packages/url/src/remove-query-args.js b/packages/url/src/remove-query-args.ts
similarity index 84%
rename from packages/url/src/remove-query-args.js
rename to packages/url/src/remove-query-args.ts
index 5d31313b08a74e..7695d2527979f1 100644
--- a/packages/url/src/remove-query-args.js
+++ b/packages/url/src/remove-query-args.ts
@@ -7,17 +7,17 @@ import { buildQueryString } from './build-query-string';
 /**
  * Removes arguments from the query string of the url
  *
- * @param {string}    url  URL.
- * @param {...string} args Query Args.
+ * @param url  URL.
+ * @param args Query Args.
  *
  * @example
  * ```js
  * const newUrl = removeQueryArgs( 'https://wordpress.org?foo=bar&bar=baz&baz=foobar', 'foo', 'bar' ); // https://wordpress.org?baz=foobar
  * ```
  *
- * @return {string} Updated URL.
+ * @return Updated URL.
  */
-export function removeQueryArgs( url, ...args ) {
+export function removeQueryArgs( url: string, ...args: string[] ): string {
 	const fragment = url.replace( /^[^#]*/, '' );
 	url = url.replace( /#.*/, '' );
 
diff --git a/packages/url/src/safe-decode-uri-component.js b/packages/url/src/safe-decode-uri-component.ts
similarity index 61%
rename from packages/url/src/safe-decode-uri-component.js
rename to packages/url/src/safe-decode-uri-component.ts
index 2bf2047803fba3..236637bf768470 100644
--- a/packages/url/src/safe-decode-uri-component.js
+++ b/packages/url/src/safe-decode-uri-component.ts
@@ -2,11 +2,11 @@
  * Safely decodes a URI component with `decodeURIComponent`. Returns the URI component unmodified if
  * `decodeURIComponent` throws an error.
  *
- * @param {string} uriComponent URI component to decode.
+ * @param uriComponent URI component to decode.
  *
- * @return {string} Decoded URI component if possible.
+ * @return Decoded URI component if possible.
  */
-export function safeDecodeURIComponent( uriComponent ) {
+export function safeDecodeURIComponent( uriComponent: string ): string {
 	try {
 		return decodeURIComponent( uriComponent );
 	} catch ( uriComponentError ) {
diff --git a/packages/url/src/safe-decode-uri.js b/packages/url/src/safe-decode-uri.ts
similarity index 72%
rename from packages/url/src/safe-decode-uri.js
rename to packages/url/src/safe-decode-uri.ts
index f5f6379e19164a..ab0830597fd0df 100644
--- a/packages/url/src/safe-decode-uri.js
+++ b/packages/url/src/safe-decode-uri.ts
@@ -2,16 +2,16 @@
  * Safely decodes a URI with `decodeURI`. Returns the URI unmodified if
  * `decodeURI` throws an error.
  *
- * @param {string} uri URI to decode.
+ * @param uri URI to decode.
  *
  * @example
  * ```js
  * const badUri = safeDecodeURI( '%z' ); // does not throw an Error, simply returns '%z'
  * ```
  *
- * @return {string} Decoded URI if possible.
+ * @return Decoded URI if possible.
  */
-export function safeDecodeURI( uri ) {
+export function safeDecodeURI( uri: string ): string {
 	try {
 		return decodeURI( uri );
 	} catch ( uriError ) {
diff --git a/packages/url/src/test/index.js b/packages/url/src/test/index.js
index 0be7c7542b71c8..9243471c8c079c 100644
--- a/packages/url/src/test/index.js
+++ b/packages/url/src/test/index.js
@@ -494,12 +494,12 @@ describe( 'getPathAndQueryString', () => {
 	beforeAll( jest.resetModules );
 	afterAll( jest.resetModules );
 	it( 'combines the results of `getPath` and `getQueryString`', () => {
-		jest.doMock( '../get-path.js', () => ( {
+		jest.doMock( '../get-path', () => ( {
 			getPath( { path } = {} ) {
 				return path;
 			},
 		} ) );
-		jest.doMock( '../get-query-string.js', () => ( {
+		jest.doMock( '../get-query-string', () => ( {
 			getQueryString( { queryString } = {} ) {
 				return queryString;
 			},