📝 Fix code style issue

* Change `var` to `let`.
* Change `function() {}` to `() => {}`.
* Use shorthand function syntax on object notation.
* Remove spaces between object notation brackets.
* Small fixes.

docs/api/screen.md

@@ -8,7 +8,7 @@ emitted (by invoking or requiring it).
 `screen` is an [EventEmitter](http://nodejs.org/api/events.html#events_class_events_eventemitter).
 
 **Note:** In the renderer / DevTools, `window.screen` is a reserved DOM
-property, so writing `var screen = require('electron').screen` will not work.
+property, so writing `let {screen} = require('electron')` will not work.
 In our examples below, we use `electronScreen` as the variable name instead.
 An example of creating a window that fills the whole screen:
 
@@ -19,7 +19,7 @@ let mainWindow;
 
 app.on('ready', () => {
   const {width, height} = electronScreen.getPrimaryDisplay().workAreaSize;
-  mainWindow = new BrowserWindow({ width, height });
+  mainWindow = new BrowserWindow({width, height});
 });
 ```
 
@@ -31,8 +31,8 @@ const {app, BrowserWindow, screen: electronScreen} = require('electron');
 let mainWindow;
 
 app.on('ready', () => {
-  var displays = electronScreen.getAllDisplays();
-  var externalDisplay = null;
+  let displays = electronScreen.getAllDisplays();
+  let externalDisplay = null;
   for (let i in displays) {
     if (displays[i].bounds.x !== 0 || displays[i].bounds.y !== 0) {
       externalDisplay = displays[i];