diff --git a/packages/i18n/README.md b/packages/i18n/README.md
index 519a2ad6006c1..50376d53c3647 100644
--- a/packages/i18n/README.md
+++ b/packages/i18n/README.md
@@ -38,8 +38,8 @@ _Related_
 
 _Parameters_
 
--   _data_ `?Object`: Locale data configuration.
--   _domain_ `?string`: Domain for which configuration applies.
+-   _data_ `[LocaleData]`: Locale data configuration.
+-   _domain_ `[string]`: Domain for which configuration applies.
 
 <a name="sprintf" href="#sprintf">#</a> **sprintf**
 
@@ -73,7 +73,7 @@ _Parameters_
 -   _single_ `string`: The text to be used if the number is singular.
 -   _plural_ `string`: The text to be used if the number is plural.
 -   _number_ `number`: The number to compare against to use either the singular or plural form.
--   _domain_ `?string`: Domain to retrieve the translated text.
+-   _domain_ `[string]`: Domain to retrieve the translated text.
 
 _Returns_
 
@@ -94,7 +94,7 @@ _Parameters_
 -   _plural_ `string`: The text to be used if the number is plural.
 -   _number_ `number`: The number to compare against to use either the singular or plural form.
 -   _context_ `string`: Context information for the translators.
--   _domain_ `?string`: Domain to retrieve the translated text.
+-   _domain_ `[string]`: Domain to retrieve the translated text.
 
 _Returns_
 
@@ -112,7 +112,7 @@ _Parameters_
 
 -   _text_ `string`: Text to translate.
 -   _context_ `string`: Context information for the translators.
--   _domain_ `?string`: Domain to retrieve the translated text.
+-   _domain_ `[string]`: Domain to retrieve the translated text.
 
 _Returns_
 
@@ -129,7 +129,7 @@ _Related_
 _Parameters_
 
 -   _text_ `string`: Text to translate.
--   _domain_ `?string`: Domain to retrieve the translated text.
+-   _domain_ `[string]`: Domain to retrieve the translated text.
 
 _Returns_
 
diff --git a/packages/i18n/src/index.js b/packages/i18n/src/index.js
index dc17951731201..c696cb9f76400 100644
--- a/packages/i18n/src/index.js
+++ b/packages/i18n/src/index.js
@@ -5,15 +5,19 @@ import Tannin from 'tannin';
 import memoize from 'memize';
 import sprintfjs from 'sprintf-js';
 
+/**
+ * @typedef {{[key: string]: any}} LocaleData
+ */
+
 /**
  * Default locale data to use for Tannin domain when not otherwise provided.
  * Assumes an English plural forms expression.
  *
- * @type {Object}
+ * @type {LocaleData}
  */
 const DEFAULT_LOCALE_DATA = {
 	'': {
-		plural_forms: ( n ) => n === 1 ? 0 : 1,
+		plural_forms: ( n ) => ( n === 1 ? 0 : 1 ),
 	},
 };
 
@@ -39,8 +43,8 @@ const i18n = new Tannin( {} );
  *
  * @see http://messageformat.github.io/Jed/
  *
- * @param {?Object} data   Locale data configuration.
- * @param {?string} domain Domain for which configuration applies.
+ * @param {LocaleData} [data]   Locale data configuration.
+ * @param {string}     [domain] Domain for which configuration applies.
  */
 export function setLocaleData( data, domain = 'default' ) {
 	i18n.data[ domain ] = {
@@ -61,13 +65,14 @@ export function setLocaleData( data, domain = 'default' ) {
  * Wrapper for Tannin's `dcnpgettext`. Populates default locale data if not
  * otherwise previously assigned.
  *
- * @param {?string} domain  Domain to retrieve the translated text.
- * @param {?string} context Context information for the translators.
- * @param {string}  single  Text to translate if non-plural. Used as fallback
- *                          return value on a caught error.
- * @param {?string} plural  The text to be used if the number is plural.
- * @param {?number} number  The number to compare against to use either the
- *                          singular or plural form.
+ * @param {string|undefined} domain   Domain to retrieve the translated text.
+ * @param {string|undefined} context  Context information for the translators.
+ * @param {string}           single   Text to translate if non-plural. Used as
+ *                                    fallback return value on a caught error.
+ * @param {string}           [plural] The text to be used if the number is
+ *                                    plural.
+ * @param {number}           [number] The number to compare against to use
+ *                                    either the singular or plural form.
  *
  * @return {string} The translated string.
  */
@@ -84,8 +89,8 @@ function dcnpgettext( domain = 'default', context, single, plural, number ) {
  *
  * @see https://developer.wordpress.org/reference/functions/__/
  *
- * @param {string}  text   Text to translate.
- * @param {?string} domain Domain to retrieve the translated text.
+ * @param {string} text     Text to translate.
+ * @param {string} [domain] Domain to retrieve the translated text.
  *
  * @return {string} Translated text.
  */
@@ -98,9 +103,9 @@ export function __( text, domain ) {
  *
  * @see https://developer.wordpress.org/reference/functions/_x/
  *
- * @param {string}  text    Text to translate.
- * @param {string}  context Context information for the translators.
- * @param {?string} domain  Domain to retrieve the translated text.
+ * @param {string} text     Text to translate.
+ * @param {string} context  Context information for the translators.
+ * @param {string} [domain] Domain to retrieve the translated text.
  *
  * @return {string} Translated context string without pipe.
  */
@@ -114,11 +119,11 @@ export function _x( text, context, domain ) {
  *
  * @see https://developer.wordpress.org/reference/functions/_n/
  *
- * @param {string}  single The text to be used if the number is singular.
- * @param {string}  plural The text to be used if the number is plural.
- * @param {number}  number The number to compare against to use either the
- *                         singular or plural form.
- * @param {?string} domain Domain to retrieve the translated text.
+ * @param {string} single   The text to be used if the number is singular.
+ * @param {string} plural   The text to be used if the number is plural.
+ * @param {number} number   The number to compare against to use either the
+ *                          singular or plural form.
+ * @param {string} [domain] Domain to retrieve the translated text.
  *
  * @return {string} The translated singular or plural form.
  */
@@ -132,12 +137,12 @@ export function _n( single, plural, number, domain ) {
  *
  * @see https://developer.wordpress.org/reference/functions/_nx/
  *
- * @param {string}  single  The text to be used if the number is singular.
- * @param {string}  plural  The text to be used if the number is plural.
- * @param {number}  number  The number to compare against to use either the
+ * @param {string} single   The text to be used if the number is singular.
+ * @param {string} plural   The text to be used if the number is plural.
+ * @param {number} number   The number to compare against to use either the
  *                          singular or plural form.
- * @param {string}  context Context information for the translators.
- * @param {?string} domain  Domain to retrieve the translated text.
+ * @param {string} context  Context information for the translators.
+ * @param {string} [domain] Domain to retrieve the translated text.
  *
  * @return {string} The translated singular or plural form.
  */
@@ -149,8 +154,8 @@ export function _nx( single, plural, number, context, domain ) {
  * Returns a formatted string. If an error occurs in applying the format, the
  * original format string is returned.
  *
- * @param {string}   format  The format of the string to generate.
- * @param {...string} args Arguments to apply to the format.
+ * @param {string}    format The format of the string to generate.
+ * @param {...string} args   Arguments to apply to the format.
  *
  * @see http://www.diveintojavascript.com/projects/javascript-sprintf
  *
diff --git a/tsconfig.json b/tsconfig.json
index 4d985031db1c4..46e4b01c2bc0b 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -22,11 +22,13 @@
 
 	},
 	"include": [
+		"./packages/i18n/**/*.js",
 		"./packages/priority-queue/**/*.js",
 		"./packages/token-list/**/*.js",
 		"./packages/url/**/*.js"
 	],
 	"exclude": [
+		"./packages/*/benchmark",
 		"./packages/**/test/**",
 		"./packages/**/build/**",
 		"./packages/**/build-module/**"