objects-classes, ch4: clarifying nuance of concise methods not being valid with 'new' operator, closes #1801

Author: getify

objects-classes/ch4.md

@@ -290,14 +290,24 @@ We've so far seen three different ways of context assignment at the function cal
 A fourth way to call a function, and assign the `this` for that invocation, is with the `new` keyword:
 
 ```js
-var point = { /* .. */ };
+var point = {
+    // ..
+
+    init: function() { /* .. */ }
+
+    // ..
+};
 
 var anotherPoint = new point.init(3,4);
 
 anotherPoint.x;     // 3
 anotherPoint.y;     // 4
 ```
 
+| TIP: |
+| :--- |
+| This example has a bit of nuance to be explained. The `init: function() { .. }` form shown here -- specifically, a function expression assigned to a property -- is required for the function to be validly called with the `new` keyword. From previous snippets, the concise method form of `init() { .. }` defines a function that *cannot* be called with `new`. |
+
 You've typically seen `new` used with `class` for creating instances. But as an underlying mechanism of the JS language, `new` is not inherently a `class` operation.
 
 In a sense, the `new` keyword hijacks a function and forces its behavior into a different mode than a normal invocation. Here are the 4 special steps that JS performs when a function is invoked with `new`: