Finished Chapter 6.9 Object Methods

notes/chap6_objects.md

@@ -462,14 +462,78 @@ What can/cannot be serialized?
 
 ## 6.9 Object Methods
 
+All JS objects (excpet explicitly created w/o prototype) inherits properties from `Object.prototype`. Hence they inherited some **primarily methods**, which are universally available.
+* `hasOwnProperty()`, `propertyIsEnumerable()`
+* `Object.create()`, `Object.keys()`
+* `toString`
+* ...
+
 ### 6.9.1 The toString() Method
 
+* `toString()` takes no arg, and returns a string that represents the value of obj.
+* Default `toString()` is not useful (e.g. shown below), each class define their own `toString()`
+
+```js
+let s = { x: 1, y: 1 }.toString();  // s evaluate to "[object Object]", without show real information
+```
+
+* We can redefine `toString()` method:
+
+```js
+let point = {
+    x: 1,
+    y: 2,
+    toString: function() { return `(${this.x}, ${this.y})`; }
+};
+
+String(point)    // => "(1, 2)": toString() is used for string conversions
+```
+
 ### 6.9.2 The toLocaleString() Method
 
+`toLocaleString()`:
+* All objs have this method
+* Purpose of this method: return a localized (vs. internationalization) string representation of the obj.
+* default `toLocaleString()` defined by Object call `toString()` directly
+* Date/Number class defin customized versions of `toLocaleString()` to format date, currency, time, etc.
+* User can create their own method:
+
+```js
+let point = {
+    x: 1000,
+    y: 2000,
+    toString: function() { return `(${this.x}, ${this.y})`; },
+    toLocaleString: function() {
+        return `(${this.x.toLocaleString()}, ${this.y.toLocaleString()})`;
+    }
+};
+point.toString()        // => "(1000, 2000)"
+point.toLocaleString()  // => "(1,000, 2,000)": note thousands separators
+```
+
 ### 6.9.3 The valueOf() Method
 
+`valueOf()`:
+* is called when JS needs to convert an obj to some non-string primitive type (e.g. number)
+* Many built-in class has its own `valueOf()`
+  * Date class define its `valueOf()` to convert dates to number, so can perform comparison
+
 ### 6.9.4 The toJSON() Method
 
+`toJSON()` method:
+* didn't get defined in `Object.prototype`;
+* `JSON.stringify()` looks for & invoke it on any object that will be serialized
+
+```js
+let point = {
+    x: 1,
+    y: 2,
+    toString: function() { return `(${this.x}, ${this.y})`; },
+    toJSON: function() { return this.toString(); }
+};
+JSON.stringify([point])   // => '["(1, 2)"]'
+```
+
 ## 6.10 Extended Object Literal Syntax
 
 ### 6.10.1 Shorthand Properties