Finished Chapter 6.3.3

notes/chap6_objects.md

@@ -146,10 +146,94 @@ function computeValue(portfolio) {
 
 ### 6.3.2 Inheritance
 
+JS obj have a set of "own properties", and they also inherit properties from prototype chain.
+
+Read properties:
+* If a property cannot be found in a JS obj, it will search one by one (bottom to top, from child to parent) through prototype chain
+
+```js
+let o = {};               // o inherits object methods from Object.prototype
+o.x = 1;                  // and it now has an own property x.
+let p = Object.create(o); // p inherits properties from o and Object.prototype
+p.y = 2;                  // and has an own property y.
+let q = Object.create(p); // q inherits properties from p, o, and...
+q.z = 3;                  // ...Object.prototype and has an own property z.
+let f = q.toString();     // toString is inherited from Object.prototype
+q.x + q.y                 // => 3; x and y are inherited from o and p
+```
+
+Write (assign) properties:
+* check prototype chain only to verify whether read-only.
+* If inherited property `x` is read-only, assignment is not allowed
+* If assignment is allowed, the property is created/set within the current object, and do not modify prototype chain
+* Only exception: if `o` inherits property `x`, and that property is an accessor property with a setter method, then the setter method is called rather than creating a new property `x` within `o`.
+
+```js
+let unitcircle = { r: 1 };         // An object to inherit from
+let c = Object.create(unitcircle); // c inherits the property r
+c.x = 1; c.y = 1;                  // c defines two properties of its own
+c.r = 2;                           // c overrides its inherited property
+unitcircle.r                       // => 1: the prototype is not affected
+```
+
 ### 6.3.3 Property Access Errors
 
+Errors during accessing property:
+* It's not error to query a property/object that does not exist. Return `undefined`
+* It's error to query property of an non-existent object. Return `TypeError`
+
+Query non-exist object
+```js
+book.subtitle    // => undefined: property doesn't exist
+```
+
+Query property of an non-exist object
+```js
+let len = book.subtitle.length; // !TypeError: undefined doesn't have length
+```
+
+Method to guard against this problem type:
+* Method 1: verbose and explicit
+```js
+let surname = undefined;
+if (book) {
+    if (book.author) {
+        surname = book.author.surname;
+    }
+}
+```
+
+* Method 2: A concise and idiomatic alternative to get surname or null or undefined
+  * Check Chap4.10.1 for short-circuiting behavior of && operator
+```js
+surname = book && book.author && book.author.surname;
+```
+
+* Method 3: Rewrite method 2 using `?.`
+```js
+let surname = book?.author?.surname;
+```
+
+Lists of tips:
+* Attempting to set property on `null` or `undefined` causes a `TypeError`
+* Attempting to set property may fail due to
+  * Some properties are read-only
+  * Some objects don't allow adding new properties
+* Error from property assignment:
+  * In strict mode (Chap 5.6.3), a TypeError is thown whenever an attempt to set a property fails.
+  * Outsie strict mode, silent when fail
+
+3 circumstances when failed to set a property `p` of obj `o`:
+1. `o` has an own property `p` that is read-only
+2. `o` has an inherited property `p` that is read-only: No way to overwrite this property
+3. `o` does not have an own property `p`; `o` does not inherit a property `p` with a setter method, and `o`’s extensible attribute (see §14.2) is `false`.
+   1. Since `p` does not already exist in `o`, and if there is no setter method to call, then `p` must be added to `o`.
+   2. But if `o` is not extensible, then no new properties can be defined on it.
+
 ## 6.4 Deleting Properties
 
+
+
 ## 6.5 Testing Properties
 
 ## 6.6 Enumerating Properties