Return function to restore a costume when it gets deleted.

src/sprites/rendered-target.js

@@ -534,12 +534,19 @@ class RenderedTarget extends Target {
     /**
      * Delete a costume by index.
      * @param {number} index Costume index to be deleted
+     * @return {?object} The costume that was deleted or null
+     * if the index was out of bounds of the costumes list or
+     * this target only has one costume.
      */
     deleteCostume (index) {
         const originalCostumeCount = this.sprite.costumes.length;
-        if (originalCostumeCount === 1) return;
+        if (originalCostumeCount === 1) return null;
 
-        this.sprite.deleteCostumeAt(index);
+        if (index < 0 || index >= originalCostumeCount) {
+            return null;
+        }
+
+        const deletedCostume = this.sprite.deleteCostumeAt(index);
 
         if (index === this.currentCostume && index === originalCostumeCount - 1) {
             this.setCostume(index - 1);
@@ -550,6 +557,7 @@ class RenderedTarget extends Target {
         }
 
         this.runtime.requestTargetsUpdate(this);
+        return deletedCostume;
     }
 
     /**

src/sprites/sprite.js

@@ -93,11 +93,10 @@ class Sprite {
     /**
      * Delete a costume by index.
      * @param {number} index Costume index to be deleted
+     * @return {?object} The deleted costume
      */
     deleteCostumeAt (index) {
-        this.costumes_ = this.costumes_
+        return this.costumes.splice(index, 1)[0];
-            .slice(0, index)
-            .concat(this.costumes_.slice(index + 1));
     }
 
     /**

src/virtual-machine.js

@@ -589,9 +589,19 @@ class VirtualMachine extends EventEmitter {
     /**
      * Delete a costume from the current editing target.
      * @param {int} costumeIndex - the index of the costume to be removed.
+     * @return {?function} A function to restore the deleted costume, or null,
+     * if no costume was deleted.
      */
     deleteCostume (costumeIndex) {
-        this.editingTarget.deleteCostume(costumeIndex);
+        const deletedCostume = this.editingTarget.deleteCostume(costumeIndex);
+        if (deletedCostume) {
+            const target = this.editingTarget;
+            return () => {
+                target.addCostume(deletedCostume);
+                this.emitTargetsUpdate();
+            };
+        }
+        return null;
     }
 
     /**