Finished Chapter 6.4 Deleting Properties

notes/chap6_objects.md

@@ -232,7 +232,59 @@ Lists of tips:
 
 ## 6.4 Deleting Properties
 
+`delte` operator removes a property from an obj. Its operand is *property access expression*
 
+`delete` & prototypes:
+* only delete own properties, cannot delete inherited ones (from prototypes).
+* deleting inherited properties need to it on prototype, which has affect all children obj.
+
+`delete` return true if:
+* delete succeeded
+* when delete had no effect (e.g. 
+  * on inherited properties
+  * non-exist properties
+* when used with an expression that's not a property access expression
+
+```js
+let o = {x: 1};    // o has own property x and inherits property toString
+
+delete o.x         // => true: deletes property x
+delete o.x         // => true: does nothing (x doesn't exist) but true anyway
+
+delete o.toString  // => true: does nothing (toString isn't o's own property)
+
+delete 1           // => true: nonsense, but true anyway
+```
+
+`delete` fails if:
+* trying to rm properties that have a `configurable` attribute of `false`.
+* trying to delete non-configurable properties of built-in objects
+  * properties of the global object created by variable declaration and function declaration
+
+`delete`'s failure in strict & non-strict mode:
+* In strict mode: causes TypeError
+* In non-strict mode: evaluate to `false` e.g. shown below
+
+```js
+// In strict mode, all these deletions throw TypeError instead of returning false
+delete Object.prototype // => false: property is non-configurable
+var x = 1;              // Declare a global variable
+delete globalThis.x     // => false: can't delete this property
+function f() {}         // Declare a global function
+delete globalThis.f     // => false: can't delete this property either
+```
+
+`delete` configurable properties of global object:
+* In non-strict mode, via directly delete property name
+```js
+globalThis.x = 1;       // Create a configurable global property (no let or var)
+delete x                // => true: this property can be deleted
+```
+* In strict mode, method above will create **SyntaxError**. Hence need to be explicit:
+```js
+delete x;               // SyntaxError in strict mode
+delete globalThis.x;    // This works
+```
 
 ## 6.5 Testing Properties