save at will (not automatically)

sudoku.js

@@ -10,6 +10,7 @@ let suggestionTimer = null
 let valueToInsert = ""
 let history = []
 let accessKeyModifiers = "AccessKey+"
+let changesToSave = false
 
 function shuffle(iterable) {
     array = Array.from(iterable)
@@ -142,6 +143,7 @@ function onclick() {
 function oninput() {
     history.push({ box: this, value: this.previousValue, placeholder: this.previousPlaceholder })
     undoButton.disabled = false
+    changesToSave = true
     if (pencilRadio.checked) {
         this.value = Array.from(new Set(this.value)).sort().join("")
         this.previousValue = ""
@@ -157,17 +159,10 @@ function oninput() {
 }
 
 function refreshBox(box) {
-    saveGame()
     checkBox(box)
     refreshUI()
 }
 
-function saveGame() {
-    let saveGame = boxes.map(box => box.value || UNKNOWN).join("")
-    localStorage[location.pathname] = saveGame
-    fixGridLink.href = saveGame
-}
-
 function checkBox(box) {
     box.neighbourhood.concat([box]).forEach(neighbour => {
         searchCandidatesOf(neighbour)
@@ -200,6 +195,7 @@ function checkBox(box) {
     } else { // Errors on grid
         box.form.reportValidity()
     }
+}
 
 function refreshUI() {
     enableRadio()
@@ -316,6 +312,20 @@ function restart() {
     }
 }
 
+function save() {
+    let saveGame = boxes.map(box => box.value || UNKNOWN).join("")
+    localStorage[location.pathname] = saveGame
+    fixGridLink.href = saveGame
+    changesToSave = false
+}
+
+window.onbeforeunload = function(event) {
+    if (changesToSave) {
+        event.preventDefault()
+        event.returnValue = ""
+    }
+}
+
 function showSuggestion() {
     const easyBoxes = boxes.filter(box => box.value == "" && box.candidates.size == 1)
     if (easyBoxes.length) {