Split move doing from event handling

takhta.html

@@ -207,6 +207,13 @@
 					return;
 				}
 
+				let ret = doMove(move);
+				if ('error' in ret) {
+					alert(ret.error);
+				}
+			}
+
+			function doMove(move) {
 				// Piece
 				let piece = move.slice(0, 1);
 				let index = 0;
@@ -230,8 +237,7 @@
 						piece = 'P';
 						break;
 					default:
-						alert('Not a valid piece: ' + piece);
+						return {error: 'Not a valid piece: ' + piece};
-						return;
 				}
 
 				// Start square
@@ -242,8 +248,7 @@
 					startLetter === '' || !'abcdefgh'.includes(startLetter) ||
 					startNumber === '' || !'12345678'.includes(startNumber)
 				) {
-					alert('Not a valid coordinate: ' + startLetter + startNumber);
+					return {error: 'Not a valid coordinate: ' + startLetter + startNumber};
-					return;
 				}
 
 				// Capturing
@@ -261,8 +266,7 @@
 					endLetter === '' || !'abcdefgh'.includes(endLetter) ||
 					endNumber === '' || !'12345678'.includes(endNumber)
 				) {
-					alert('Not a valid coordinate: ' + endLetter + endNumber);
+					return {error: 'Not a valid coordinate: ' + endLetter + endNumber};
-					return;
 				}
 
 				// Promotion
@@ -277,16 +281,13 @@
 							promotion = move.slice(index, index + 1);
 							break;
 						case 'K':
-							alert('Cannot promote a piece to king');
+							return {error: 'Cannot promote a piece to king'};
-							return;
 							break;
 						case 'P':
-							alert('Cannot promote a piece to pawn');
+							return {error: 'Cannot promote a piece to pawn'};
-							return;
 							break;
 						default:
-							alert('Unrecognized piece: ' + move.slice(index, index + 1));
+							return {error: 'Unrecognized piece: ' + move.slice(index, index + 1)};
-							return;
 					}
 					index++;
 				}
@@ -303,8 +304,7 @@
 						// TODO: Checkmate'
 						break;
 					default:
-						alert('Unexpected contents at the end of the move: ' + move.slice(index));
+						return {error: 'Unexpected contents at the end of the move: ' + move.slice(index)};
-						return;
 				}
 
 				// End of parsing, start of move construction
@@ -321,20 +321,17 @@
 
 				// Are we moving the piece onto itself?
 				if (startLetter === endLetter && startNumber === endNumber) {
-					alert('Start and end squares of the move cannot be identical');
+					return {error: 'Start and end squares of the move cannot be identical'};
-					return;
 				}
 
 				// Are we moving the piece from where it is located?
 				let startSquare = document.getElementById(startLetter + startNumber);
 				if (startSquare.childNodes.length === 0) {
-					alert('No piece on square ' + startLetter + startNumber);
+					return {error: 'No piece on square ' + startLetter + startNumber};
-					return;
 				}
 				if (startSquare.childNodes[0].data !== renderedPiece) {
 					let pieceNames = {'K': 'king', 'Q': 'vizier', 'R': 'rook', 'B': 'elephant', 'N': 'horse', 'P': 'pawn'};
-					alert((whiteMove ? 'White' : 'Black') + ' ' + pieceNames[piece] + ' not on square ' + startLetter + startNumber);
+					return {error: (whiteMove ? 'White' : 'Black') + ' ' + pieceNames[piece] + ' not on square ' + startLetter + startNumber};
-					return;
 				}
 
 				// TODO: Check piece is allowed to move like that
@@ -342,12 +339,10 @@
 				// Can the piece move to this location?
 				let endSquare = document.getElementById(endLetter + endNumber);
 				if (endSquare.childNodes.length !== 0 && !captures) {
-					alert('Square ' + endLetter + endNumber + ' is occupied, did you mean to capture?');
+					return {error: 'Square ' + endLetter + endNumber + ' is occupied, did you mean to capture?'};
-					return;
 				}
 				if (endSquare.childNodes.length === 0 && captures) {
-					alert('Square ' + endLetter + endNumber + ' empty even though capture was specified');
+					return {error: 'Square ' + endLetter + endNumber + ' empty even though capture was specified'};
-					return;
 				}
 
 				// Record our alterations to the table
@@ -377,6 +372,8 @@
 
 				// Empty input field
 				document.getElementById('move').value = '';
+
+				return {}
 			}
 
 			function undoMove() {