diff --git a/lib/chessboard.js b/lib/chessboard.js
index 95125fb..f821375 100644
--- a/lib/chessboard.js
+++ b/lib/chessboard.js
@@ -71,17 +71,6 @@ function throttle(f, interval) {
   };
 }
 
-// function debounce (f, interval, scope) {
-//   const timeout = 0
-//   return function (_args) {
-//     window.clearTimeout(timeout)
-//     const args = arguments
-//     timeout = window.setTimeout(function () {
-//       f.apply(scope, args)
-//     }, interval)
-//   }
-// }
-
 function uuid() {
   return 'xxxx-xxxx-xxxx-xxxx-xxxx-xxxx-xxxx-xxxx'.replace(/x/g, function () {
     const r = (Math.random() * 16) | 0;
@@ -123,38 +112,24 @@ function interpolateTemplate(str, obj) {
 // Predicates
 // ---------------------------------------------------------------------------
 
-function isString(s) {
-  return typeof s === 'string';
-}
-
-function isFunction(f) {
-  return typeof f === 'function';
-}
-
-function isInteger(n) {
-  return typeof n === 'number' &&
-    isFinite(n) &&
-    Math.floor(n) === n;
-}
-
 function isPlainObject(o) {
   return Object.prototype.toString.call(o) === '[object Object]';
 }
 
 function validAnimationSpeed(speed) {
   if (speed === 'fast' || speed === 'slow') return true;
-  if (!isInteger(speed)) return false;
+  if (!Number.isInteger(speed)) return false;
   return speed >= 0;
 }
 
 function validThrottleRate(rate) {
-  return isInteger(rate) &&
+  return Number.isInteger(rate) &&
     rate >= 1;
 }
 
 function validMove(move) {
   // move should be a string
-  if (!isString(move)) return false;
+  if (typeof move !== 'string') return false;
 
   // move should be in the form of "e2-e4", "f6-d5"
   const squares = move.split('-');
@@ -164,15 +139,20 @@ function validMove(move) {
 }
 
 function validSquare(square) {
-  return isString(square) && square.search(/^[a-h][1-8]$/) !== -1;
+  return typeof square === 'string' && square.search(/^[a-h][1-8]$/) !== -1;
 }
 
 function validPieceCode(code) {
-  return isString(code) && code.search(/^[bw][KQRNBP]$/) !== -1;
+  return typeof code === 'string' && code.search(/^[bw][KQRNBP]$/) !== -1;
 }
 
+/**
+ *
+ * @param {string} [fen]
+ * @returns {boolean}
+ */
 function validFen(fen) {
-  if (!isString(fen)) return false;
+  if (typeof fen !== 'string') return false;
 
   // cut off any move, castling, etc info from the end
   // we're only interested in position information
@@ -242,17 +222,20 @@ function pieceCodeToFen(piece) {
   return pieceCodeLetters[1].toLowerCase();
 }
 
-// convert FEN string to position object
-// returns false if the FEN string is invalid
+/**
+ * Convert FEN string to position object. Returns `false` if the FEN string is invalid
+ * @param {string} [fen]
+ * @returns {false | Record<string, string>}
+ */
 function fenToObj(fen) {
-  if (!validFen(fen)) return false;
+  if (!fen || !validFen(fen)) return false;
 
   // cut off any move, castling, etc info from the end
   // we're only interested in position information
   fen = fen.replace(/ .+$/, '');
 
   const rows = fen.split('/');
-  const position = {};
+  const position = /** @type {Record<string, string>} */({});
 
   let currentRow = 8;
   for (let i = 0; i < 8; i++) {
@@ -279,8 +262,13 @@ function fenToObj(fen) {
   return position;
 }
 
-// position object to FEN string
-// returns false if the obj is not a valid position object
+//
+//
+/**
+ * Converts position object to FEN string. Returns false if the obj is not a valid position object
+ * @param {object} obj
+ * @returns {false | string}
+ */
 function objToFen(obj) {
   if (!validPositionObject(obj)) return false;
 
@@ -313,6 +301,10 @@ function objToFen(obj) {
   return fen;
 }
 
+/**
+ * @param {string} fen
+ * @returns {string}
+ */
 function squeezeFenEmptySquares(fen) {
   return fen.replace(/11111111/g, '8')
     .replace(/1111111/g, '7')
@@ -323,6 +315,10 @@ function squeezeFenEmptySquares(fen) {
     .replace(/11/g, '2');
 }
 
+/**
+ * @param {string} fen
+ * @returns {string}
+ */
 function expandFenEmptySquares(fen) {
   return fen.replace(/8/g, '11111111')
     .replace(/7/g, '1111111')
@@ -483,7 +479,7 @@ function expandConfig(config) {
 
   // default piece theme is wikipedia
   if (!Object.prototype.hasOwnProperty.call(config, 'pieceTheme') ||
-    (!isString(config.pieceTheme) && !isFunction(config.pieceTheme))) {
+    (typeof config.pieceTheme !== 'string' && typeof config.pieceTheme !== 'function')) {
     config.pieceTheme = 'img/chesspieces/wikipedia/{piece}.png';
   }
 
@@ -516,14 +512,13 @@ function checkContainerArg(containerElOrString) {
   }
 
   // convert containerEl to query selector if it is a string
-  if (isString(containerElOrString) &&
-    containerElOrString.charAt(0) !== '#') {
+  if (typeof containerElOrString === 'string' && containerElOrString.startsWith('#') === false) {
     containerElOrString = '#' + containerElOrString;
   }
 
   // containerEl must be something that becomes a NodeList of size 1
-  const $container = /** @type {NodeListOf<HTMLElement>} */(document.querySelectorAll(containerElOrString));
-  if ($container.length !== 1) {
+  const container = /** @type {NodeListOf<HTMLElement>} */(document.querySelectorAll(containerElOrString));
+  if (container.length !== 1) {
     const errorMsg2 = 'Chessboard Error 1003: ' +
       'The first argument to Chessboard() must be the ID of a DOM node, ' +
       'an ID query selector, or a single DOM node.' +
@@ -533,7 +528,7 @@ function checkContainerArg(containerElOrString) {
     return false;
   }
 
-  return $container.item(0);
+  return container.item(0);
 }
 
 // ---------------------------------------------------------------------------
@@ -586,6 +581,9 @@ class Chessboard {
   #sparePiecesElsIds = {};
   #squareElsIds = {};
   #squareElsOffsets = {};
+  /**
+   * @type {number}
+   */
   #squareSize = 16;
 
   constructor(containerElOrString, config) {
@@ -734,14 +732,21 @@ class Chessboard {
   position(position, useAnimation) {
     if (!position) {
       return deepCopy(this.#currentPosition);
-    } else if (isString(position) && position.toLowerCase() === 'fen') {
-      // get position as FEN
-      return objToFen(this.#currentPosition);
     }
 
-    if (isString(position) && position.toLowerCase() === 'start') {
-      // start position
-      position = deepCopy(START_POSITION);
+    const positionIsString = typeof position === 'string';
+
+    if (positionIsString) {
+      const positionLowerCase = position.toLowerCase();
+
+      if (positionLowerCase === 'fen') {
+        // get position as FEN
+        return objToFen(this.#currentPosition);
+      }
+      if (positionLowerCase === 'start') {
+        // start position
+        position = deepCopy(START_POSITION);
+      }
     }
 
     // convert FEN to position object
@@ -831,7 +836,7 @@ class Chessboard {
         this.#draggedPiece.style.setProperty('display', 'none');
 
         // execute their onSnapEnd function
-        if (isFunction(this.#config.onSnapEnd)) {
+        if (typeof this.#config.onSnapEnd === 'function') {
           this.#config.onSnapEnd(this.#draggedPieceSource, square, this.#draggedPiece);
         }
       }
@@ -876,7 +881,7 @@ class Chessboard {
     }
 
     // custom function
-    if (isFunction(this.#config.showErrors)) {
+    if (typeof this.#config.showErrors === 'function') {
       this.#config.showErrors(code, msg, obj);
     }
   }
@@ -961,11 +966,11 @@ class Chessboard {
   }
 
   #buildPieceImgSrc(piece) {
-    if (isFunction(this.#config.pieceTheme)) {
+    if (typeof this.#config.pieceTheme === 'function') {
       return this.#config.pieceTheme(piece);
     }
 
-    if (isString(this.#config.pieceTheme)) {
+    if (typeof this.#config.pieceTheme === 'string') {
       return interpolateTemplate(this.#config.pieceTheme, { piece });
     }
 
@@ -982,7 +987,7 @@ class Chessboard {
 
   #buildPieceHTML(piece, hidden, id) {
     let html = '<img src="' + this.#buildPieceImgSrc(piece) + '" ';
-    if (isString(id) && id !== '') {
+    if (typeof id === 'string' && id.length > 0) {
       html += 'id="' + id + '" ';
     }
     html += 'alt="" ' +
@@ -1043,7 +1048,7 @@ class Chessboard {
       $animatedPiece.remove();
 
       // run complete function
-      if (isFunction(completeFn)) {
+      if (typeof completeFn === 'function') {
         completeFn();
       }
     };
@@ -1075,7 +1080,7 @@ class Chessboard {
       $animatedPiece.remove();
 
       // run complete function
-      if (isFunction(completeFn)) {
+      if (typeof completeFn === 'function') {
         completeFn();
       }
     };
@@ -1094,7 +1099,7 @@ class Chessboard {
       this.#drawPositionInstant();
 
       // run their onMoveEnd function
-      if (isFunction(this.#config.onMoveEnd)) {
+      if (typeof this.#config.onMoveEnd === 'function') {
         this.#config.onMoveEnd(deepCopy(oldPos), deepCopy(newPos));
       }
     };
@@ -1246,7 +1251,7 @@ class Chessboard {
     if (oldFen === newFen) return;
 
     // run their onChange function
-    if (isFunction(this.#config.onChange)) {
+    if (typeof this.#config.onChange === 'function') {
       this.#config.onChange(oldPos, newPos);
     }
 
@@ -1314,7 +1319,7 @@ class Chessboard {
         this.#draggedPiece.style.setProperty('display', 'none');
 
         // run their onSnapbackEnd function
-        if (isFunction(this.#config.onSnapbackEnd)) {
+        if (typeof this.#config.onSnapbackEnd === 'function') {
           this.#config.onSnapbackEnd(
             this.#draggedPiece,
             this.#draggedPieceSource,
@@ -1351,7 +1356,7 @@ class Chessboard {
   #beginDraggingPiece(source, piece, x, y) {
     // run their custom onDragStart function
     // their custom onDragStart function can cancel drag start
-    if (isFunction(this.#config.onDragStart) &&
+    if (typeof this.#config.onDragStart === 'function' &&
       this.#config.onDragStart(source, piece, deepCopy(this.#currentPosition), this.#currentOrientation) === false) {
       return;
     }
@@ -1411,7 +1416,7 @@ class Chessboard {
     }
 
     // run onDragMove
-    if (isFunction(this.#config.onDragMove)) {
+    if (typeof this.#config.onDragMove === 'function') {
       this.#config.onDragMove(
         location,
         this.#draggedPieceLocation,
@@ -1437,7 +1442,7 @@ class Chessboard {
     }
 
     // run their onDrop function, which can potentially change the drop action
-    if (isFunction(this.#config.onDrop)) {
+    if (typeof this.#config.onDrop === 'function') {
       const newPosition = deepCopy(this.#currentPosition);
 
       // source piece is a spare piece and position is off the board
@@ -1598,7 +1603,7 @@ class Chessboard {
     if (this.#isDragging) return;
 
     // exit if they did not provide a onMouseoverSquare function
-    if (!isFunction(this.#config.onMouseoverSquare)) return;
+    if (typeof this.#config.onMouseoverSquare !== 'function') return;
 
     // get the square
     const square = /** @type {HTMLElement | null} */(evt.currentTarget)?.getAttribute('data-square');
@@ -1622,7 +1627,7 @@ class Chessboard {
     if (this.#isDragging) return;
 
     // exit if they did not provide an onMouseoutSquare function
-    if (!isFunction(this.#config.onMouseoutSquare)) return;
+    if (typeof this.#config.onMouseoutSquare !== 'function') return;
 
     // get the square
     const square = document.getElementById(evt.currentTarget)?.getAttribute('data-square');