diff --git a/Gruntfile.js b/Gruntfile.js
index 4a5adcf..166e196 100644
--- a/Gruntfile.js
+++ b/Gruntfile.js
@@ -16,7 +16,7 @@ module.exports = grunt => {
         },
         focus: {
             dev: {
-                include: ["css", "html", "js"],
+                include: ["css", "html", "ts"],
             },
         },
         replace: {
@@ -51,23 +51,24 @@ module.exports = grunt => {
                 tasks: ["copy:html"],
             },
             js: {
-                files: ["src/main/**/*.js"],
+                files: ["src/main/**/*.ts"],
                 tasks: ["webpack:dev", "replace:dev"],
             },
         },
         webpack: {
             options: {
-                entry: "./src/main/js/main.js",
+                entry: "./src/main/js/Main.ts",
                 module: {
                     rules: [
                         {
-                            test: /\.js$/,
+                            test: /\.ts$/,
+                            use: "ts-loader",
                             exclude: /node_modules/,
                         },
                     ],
                 },
                 resolve: {
-                    extensions: [".js"],
+                    extensions: [".ts"],
                 },
                 output: {
                     filename: "bundle.js",
diff --git a/package-lock.json b/package-lock.json
index 9aded68..404a828 100644
Binary files a/package-lock.json and b/package-lock.json differ
diff --git a/package.json b/package.json
index 88fb929..e59b826 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
     "name": "doomsday",
-    "version": "1.3.19",
+    "version": "2.0.0",
     "description": "Test your mastery of Conway's Doomsday rule.",
     "author": "Florine W. Dekker",
     "browser": "dist/bundle.js",
@@ -24,7 +24,9 @@
         "grunt-focus": "^1.0.0",
         "grunt-text-replace": "^0.4.0",
         "grunt-webpack": "^5.0.0",
-        "webpack": "^5.69.1",
+        "ts-loader": "^9.2.7",
+        "typescript": "^4.6.2",
+        "webpack": "^5.70.0",
         "webpack-cli": "^4.9.2"
     }
 }
diff --git a/src/main/js/main.js b/src/main/js/Main.ts
similarity index 75%
rename from src/main/js/main.js
rename to src/main/js/Main.ts
index f76f6cb..52cf505 100644
--- a/src/main/js/main.js
+++ b/src/main/js/Main.ts
@@ -1,4 +1,4 @@
-// noinspection JSUnresolvedVariable
+// @ts-ignore
 const {$, doAfterLoad, footer, header, nav} = window.fwdekker;
 
 
@@ -8,7 +8,7 @@ const {$, doAfterLoad, footer, header, nav} = window.fwdekker;
  * @param min the lower bound of permissible values
  * @param max the upper bound of permissible values
  */
-function generateRandom(min, max) {
+function generateRandom(min: number, max: number) {
     return Math.floor(Math.random() * (max - min + 1) + min);
 }
 
@@ -17,20 +17,30 @@ function generateRandom(min, max) {
  * An input that can be validated.
  *
  * In particular, the century, year, and day inputs of the Doomsday test.
- *
- * @property input {HTMLInputElement} the input that is validatable
- * @property titleLabel {HTMLElement} the label of which to update the text
- * @property button {HTMLButtonElement} the submission button that activates validation
  */
 class ValidatableInput {
+    /**
+     * The input that is validatable.
+     */
+    readonly input: HTMLInputElement
+    /**
+     * The label of which to update the text.
+     */
+    readonly titleLabel: HTMLElement
+    /**
+     * The submission button that activates validation.
+     */
+    readonly button: HTMLButtonElement
+
+
     /**
      * Constructs a new validatable input and registers event listeners.
      *
-     * @param input {HTMLInputElement} the input that is validatable
-     * @param titleLabel {HTMLElement} the label of which to update the text
-     * @param button {HTMLButtonElement} the submission button that activates validation
+     * @param input the input that is validatable
+     * @param titleLabel the label of which to update the text
+     * @param button the submission button that activates validation
      */
-    constructor(input, titleLabel, button) {
+    constructor(input: HTMLInputElement, titleLabel: HTMLElement, button: HTMLButtonElement) {
         this.input = input;
         this.titleLabel = titleLabel;
         this.button = button;
@@ -49,7 +59,7 @@ class ValidatableInput {
     /**
      * Handles the user submitting the input.
      */
-    onSubmit() {
+    onSubmit(): void {
         this.input.dataset["entered"] = "true";
 
         if (this.isValid(this.input.value)) {
@@ -67,10 +77,10 @@ class ValidatableInput {
      *
      * This method **must** be implemented by subclasses.
      *
-     * @param value {string} the value of the input to validate
-     * @return {boolean} `true` if and only if the input is valid
+     * @param value the value of the input to validate
+     * @return `true` if and only if the input is valid
      */
-    isValid(value) {
+    isValid(value: string): boolean {
         throw new Error("Implement this method.");
     }
 
@@ -79,7 +89,7 @@ class ValidatableInput {
      *
      * This method **must** be implemented by subclasses.
      */
-    onValidInput() {
+    onValidInput(): void {
         throw new Error("Implement this method.");
     }
 
@@ -88,7 +98,7 @@ class ValidatableInput {
      *
      * This method **must** be implemented by subclasses.
      */
-    onInvalidInput() {
+    onInvalidInput(): void {
         throw new Error("Implement this method.");
     }
 
@@ -96,7 +106,7 @@ class ValidatableInput {
     /**
      * Resets the input, title, and error message to their initial state, and removes the value from the input.
      */
-    reset() {
+    reset(): void {
         this.input.value = "";
         this.input.dataset["entered"] = "false";
 
@@ -110,7 +120,7 @@ class ValidatableInput {
     /**
      * Marks the input as invalid.
      */
-    showError() {
+    showError(): void {
         this.input.setCustomValidity("Incorrect");
 
         this.titleLabel.classList.remove("success-message");
@@ -123,7 +133,7 @@ class ValidatableInput {
     /**
      * Marks the input as valid.
      */
-    showSuccess() {
+    showSuccess(): void {
         this.input.setCustomValidity("");
 
         this.titleLabel.classList.remove("error-message");
@@ -138,7 +148,7 @@ class ValidatableInput {
      *
      * Does nothing by default. Implement this method to make it do something.
      */
-    updateTitle() {
+    updateTitle(): void {
         // Do nothing
     }
 
@@ -146,7 +156,7 @@ class ValidatableInput {
     /**
      * Focuses the input element.
      */
-    selectInput() {
+    selectInput(): void {
         this.input.select();
     }
 }
@@ -155,46 +165,60 @@ class ValidatableInput {
  * A wrapper around a `<details>` element that persists the state in local storage.
  */
 class ToggleableSection {
+    /**
+     * The name to identify this component with in persistent storage.
+     *
+     * @private
+     */
+    private readonly name: string;
+    /**
+     * The element that can be toggled.
+     *
+     * @private
+     */
+    private readonly details: HTMLDetailsElement;
+
+
     /**
      * Constructs a new `ToggleableSection`.
      *
-     * @param name {string} the name to identify this component with in persistent storage
-     * @param details {HTMLDetailsElement} the element that can be toggled
+     * @param name the name to identify this component with in persistent storage
+     * @param details the element that can be toggled
      */
-    constructor(name, details) {
-        this._name = name;
-        this._details = details;
-        this._details.addEventListener("toggle", () => this.onToggle(this.isOpened()));
+    constructor(name: string, details: HTMLDetailsElement) {
+        this.name = name;
+        this.details = details;
+        this.details.addEventListener("toggle", () => this.onToggle(this.isOpened()));
 
-        this._loadToggle();
+        this.loadToggle();
     }
 
 
     /**
      * Returns `true` if and only if the component is currently open.
      *
-     * @return {boolean} `true` if and only if the component is currently open.
+     * @return `true` if and only if the component is currently open.
      */
-    isOpened() {
-        return !!this._details.open;
+    isOpened(): boolean {
+        return this.details.open;
     }
 
     /**
      * Opens or closes the component.
      *
-     * @param isOpened {boolean} whether to open the component
+     * @param isOpened whether to open the component
      */
-    setOpened(isOpened) {
-        this._details.open = isOpened;
+    setOpened(isOpened: boolean): void {
+        this.details.open = isOpened;
     }
 
     /**
      * This method is invoked whenever the component is toggled.
      *
-     * @param isOpened {boolean} the new state of the component
+     * @param isOpened the new state of the component
      */
-    onToggle(isOpened) {
-        this._storeToggle();
+    onToggle(isOpened: boolean): void {
+        this.storeToggle();
     }
 
 
@@ -203,8 +227,8 @@ class ToggleableSection {
      *
      * @private
      */
-    _storeToggle() {
-        localStorage.setItem(`/tools/doomsday//toggle-${this._name}`, "" + this.isOpened());
+    private storeToggle(): void {
+        localStorage.setItem(`/tools/doomsday//toggle-${this.name}`, "" + this.isOpened());
     }
 
     /**
@@ -212,11 +236,11 @@ class ToggleableSection {
      *
      * @private
      */
-    _loadToggle() {
-        const target = localStorage.getItem(`/tools/doomsday//toggle-${this._name}`);
+    private loadToggle(): void {
+        const target = localStorage.getItem(`/tools/doomsday//toggle-${this.name}`);
         if (target === null) {
             this.setOpened(true);
-            this._storeToggle();
+            this.storeToggle();
             return;
         }
 
@@ -226,16 +250,20 @@ class ToggleableSection {
 
 /**
  * A wrapper around the good ol' `Date` class that provides a bunch of useful Doomsday-specific methods.
- *
- * @property {Date} date the underlying date
  */
 class DoomsdayDate {
+    /**
+     * The underlying date.
+     */
+    readonly date: Date;
+
+
     /**
      * Wraps a `DoomsdayDate` around the given date.
      *
-     * @param date {Date} the date to be wrapped
+     * @param date the date to be wrapped
      */
-    constructor(date) {
+    constructor(date: Date) {
         this.date = date;
     }
 
@@ -243,18 +271,18 @@ class DoomsdayDate {
     /**
      * Returns the number of this `DoomsdayDate`'s century.
      *
-     * @return {number} the number of this `DoomsdayDate`'s century
+     * @return the number of this `DoomsdayDate`'s century
      */
-    getCentury() {
+    getCentury(): number {
         return Math.floor(this.date.getFullYear() / 100);
     }
 
     /**
      * Returns the day of the week of the anchor of this `DoomsdayDate`'s century.
      *
-     * @return {string} the day of the week of the anchor of this `DoomsdayDate`'s century
+     * @return the day of the week of the anchor of this `DoomsdayDate`'s century
      */
-    getCenturyAnchorString() {
+    getCenturyAnchorString(): string {
         const centuryAnchorNumber = (5 * (this.getCentury() % 4)) % 7 + 2;
         return DoomsdayDate.getWeekDayOf(centuryAnchorNumber);
     };
@@ -262,9 +290,9 @@ class DoomsdayDate {
     /**
      * Returns the day of the week of the anchor day of this `DoomsdayDate`'s year.
      *
-     * @return {string} the day of the week of the anchor day of this `DoomsdayDate`'s year
+     * @return the day of the week of the anchor day of this `DoomsdayDate`'s year
      */
-    getYearAnchorString() {
+    getYearAnchorString(): string {
         const anchorDate = new Date(this.date);
         anchorDate.setDate(4); // 4th
         anchorDate.setMonth(3); // April
@@ -275,9 +303,9 @@ class DoomsdayDate {
     /**
      * Returns the day of the week of this `DoomsdayDate`.
      *
-     * @return {string} the day of the week of this `DoomsdayDate`
+     * @return the day of the week of this `DoomsdayDate`
      */
-    getWeekdayString() {
+    getWeekdayString(): string {
         return DoomsdayDate.getWeekDayOf(this.date);
     };
 
@@ -285,10 +313,10 @@ class DoomsdayDate {
     /**
      * Returns the week day of [date].
      *
-     * @param date {Date|number} the date to get the week day of; if it is a `number`, then 0 corresponds to Sunday
-     * @return {string} the name of the week day corresponding to [date]
+     * @param date the date to get the week day of; if it is a `number`, then 0 corresponds to Sunday
+     * @return the name of the week day corresponding to [date]
      */
-    static getWeekDayOf(date) {
+    static getWeekDayOf(date: Date|number): string {
         if (date instanceof Date) {
             return date.toLocaleString("en-US", {weekday: "long"});
         } else {
@@ -307,19 +335,21 @@ class DoomsdayDate {
                     return "Friday";
                 case 6:
                     return "Saturday";
+                default:
+                    throw new Error(`Unexpected weekday number '${date}'.`);
             }
         }
     };
 
     /**
-     * Returns the day of the week corresponding to the given string.
+     * Returns the day of the week corresponding to `dayString`, or an empty string if no day was recognized.
      *
      * This is a convenience method for interpreting (incomplete) user inputs.
      *
-     * @param dayString {string} the day of the week to expand
-     * @return {string} the day of the week corresponding to the given string
+     * @param dayString the day of the week to expand
+     * @return the day of the week corresponding to `dayString`, or an empty string if no day was recognized
      */
-    static expandDayString(dayString) {
+    static expandDayString(dayString: string): string {
         dayString = dayString.toLowerCase();
         if (dayString.startsWith("m"))
             return "Monday";
@@ -336,7 +366,7 @@ class DoomsdayDate {
         else if (dayString.startsWith("su"))
             return "Sunday";
         else
-            return undefined;
+            return "";
     }
 
     /**
@@ -372,17 +402,17 @@ doAfterLoad(() => {
 
 
     // Initialize quiz
-    let quizDate;
+    let quizDate: DoomsdayDate;
 
     const centuryDetails = new class extends ToggleableSection {
-        onToggle(isOpened) {
+        onToggle(isOpened: boolean): void {
             super.onToggle(isOpened);
             if (isOpened) centuryInput.selectInput();
             centuryInput.updateTitle();
         }
     }("century", $("#century-details"));
     const yearDetails = new class extends ToggleableSection {
-        onToggle(isOpened) {
+        onToggle(isOpened: boolean): void {
             super.onToggle(isOpened);
             if (isOpened) yearInput.selectInput();
             yearInput.updateTitle();
@@ -390,7 +420,7 @@ doAfterLoad(() => {
     }("year", $("#year-details"));
 
     const centuryInput = new class extends ValidatableInput {
-        isValid(value) {
+        isValid(value: string): boolean {
             console.log("# Validate century");
             console.log(`Input:    ${value}`);
             console.log(`Expanded: ${DoomsdayDate.expandDayString(value)}`);
@@ -398,7 +428,7 @@ doAfterLoad(() => {
             return DoomsdayDate.expandDayString(value) === quizDate.getCenturyAnchorString();
         }
 
-        onValidInput() {
+        onValidInput(): void {
             this.input.value = DoomsdayDate.expandDayString(this.input.value);
             if (yearDetails.isOpened())
                 yearInput.selectInput();
@@ -418,7 +448,7 @@ doAfterLoad(() => {
         }
     }($("#century-input"), $("#century-title-label"), $("#century-submit"));
     const yearInput = new class extends ValidatableInput {
-        isValid(value) {
+        isValid(value: string): boolean {
             console.log("# Validate year");
             console.log(`Input:    ${value}`);
             console.log(`Expanded: ${DoomsdayDate.expandDayString(value)}`);
@@ -443,7 +473,7 @@ doAfterLoad(() => {
         }
     }($("#year-input"), $("#year-title-label"), $("#year-submit"));
     const dayInput = new class extends ValidatableInput {
-        isValid(value) {
+        isValid(value: string): boolean {
             console.log("# Validate day");
             console.log(`Input:    ${value}`);
             console.log(`Expanded: ${DoomsdayDate.expandDayString(value)}`);
@@ -461,7 +491,7 @@ doAfterLoad(() => {
         }
 
         updateTitle() {
-            this.titleLabel.innerText = `Weekday of ${quizDate.date.toISOString().substr(0, 10)}?`;
+            this.titleLabel.innerText = `Weekday of ${quizDate.date.toISOString().substring(0, 10)}?`;
         }
     }($("#day-input"), $("#day-title-label"), $("#day-submit"));
 
@@ -479,7 +509,7 @@ doAfterLoad(() => {
     function reloadQuiz() {
         quizDate = DoomsdayDate.random();
         console.log("# Reset");
-        console.log(`New date:  ${quizDate.date.toISOString().substr(0, 10)}`);
+        console.log(`New date:  ${quizDate.date.toISOString().substring(0, 10)}`);
         console.log(`           ${quizDate.date}`);
         console.log(`Century#:  ${quizDate.getCentury()}`);
         console.log(`Century:   ${quizDate.getCenturyAnchorString()}`);
diff --git a/tsconfig.json b/tsconfig.json
new file mode 100644
index 0000000..69c22e6
--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,11 @@
+{
+    "compilerOptions": {
+        "target": "es6",
+        "strict": true,
+        "rootDir": "./src/main/js/",
+        "outDir": "./dist/js/"
+    },
+    "include": [
+        "src/main/js/**/*.ts"
+    ]
+}