Add 'will-prevent-unload' event.

spec/api-web-contents-spec.js

@@ -543,6 +543,31 @@ describe('webContents module', function () {
     })
   })
 
+  describe('will-prevent-unload event', function () {
+    it('does not emit if beforeunload returns undefined', function (done) {
+      w.once('closed', function () {
+        done()
+      })
+      w.webContents.on('will-prevent-unload', function (e) {
+        assert.fail('should not have fired')
+      })
+      w.loadURL('file://' + path.join(fixtures, 'api', 'close-beforeunload-undefined.html'))
+    })
+
+    it('emits if beforeunload returns false', (done) => {
+      w.webContents.on('will-prevent-unload', () => {
+        done()
+      })
+      w.loadURL('file://' + path.join(fixtures, 'api', 'close-beforeunload-false.html'))
+    })
+
+    it('supports calling preventDefault on will-prevent-unload events', function (done) {
+      ipcRenderer.send('prevent-next-will-prevent-unload', w.webContents.id)
+      w.once('closed', () => done())
+      w.loadURL('file://' + path.join(fixtures, 'api', 'close-beforeunload-false.html'))
+    })
+  })
+
   describe('destroy()', () => {
     let server
 

spec/static/main.js

@@ -266,6 +266,10 @@ ipcMain.on('prevent-next-will-attach-webview', (event) => {
   event.sender.once('will-attach-webview', event => event.preventDefault())
 })
 
+ipcMain.on('prevent-next-will-prevent-unload', (event, id) => {
+  webContents.fromId(id).once('will-prevent-unload', event => event.preventDefault())
+})
+
 ipcMain.on('disable-node-on-next-will-attach-webview', (event, id) => {
   event.sender.once('will-attach-webview', (event, webPreferences, params) => {
     params.src = `file://${path.join(__dirname, '..', 'fixtures', 'pages', 'c.html')}`