Finished Chapter 6.5: Testing Properties

notes/chap6_objects.md

@@ -288,6 +288,60 @@ delete globalThis.x;    // This works
 
 ## 6.5 Testing Properties
 
+JS obj = sets of properties. We want to test membership in the set using:
+* `in` operator
+* `hasOwnProperty()` method
+* `propertyIsEnumerable()` method
+* querying property
+
+`in` operator return true if object has an own property or an inherited property **by that name**:
+```js
+let o = { x: 1 };
+"x" in o         // => true: o has an own property "x"
+"y" in o         // => false: o doesn't have a property "y"
+"toString" in o  // => true: o inherits a toString property
+```
+
+`hasOwnProperty()` method of an obj return:
+* `true` if obj has an own property with the given name
+* `false` for inherited properties
+```js
+let o = { x: 1 };
+o.hasOwnProperty("x")        // => true: o has an own property x
+o.hasOwnProperty("y")        // => false: o doesn't have a property y
+o.hasOwnProperty("toString") // => false: toString is an inherited property
+```
+
+`propertyIsEnumerable()` refines `hasOwnProperty()`. It returns:
+* `true` if named property is not inherited and its *enumerable* attribute is `true`
+  * Properties created by normal JS code are enumerable unless specified
+```js
+let o = { x: 1 };
+o.propertyIsEnumerable("x")  // => true: o has an own enumerable property x
+o.propertyIsEnumerable("toString")  // => false: not an own property
+Object.prototype.propertyIsEnumerable("toString") // => false: not enumerable
+```
+
+Qeurying method is simple by using `!=` to make sure whether property is undefined
+```js
+let o = { x: 1 };
+o.x !== undefined        // => true: o has a property x
+o.y !== undefined        // => false: o doesn't have a property y
+o.toString !== undefined // => true: o inherits a toString property
+```
+
+Simple querying method vs. `in` operator:
+* `in` operator can tell whether a property does not exit (`null`) or exit but not defined (`undefined`)
+```js
+let o = { x: undefined };  // Property is explicitly set to undefined
+o.x !== undefined          // => false: property exists but is undefined
+o.y !== undefined          // => false: property doesn't even exist
+"x" in o                   // => true: the property exists
+"y" in o                   // => false: the property doesn't exist
+delete o.x;                // Delete the property x
+"x" in o                   // => false: it doesn't exist anymore
+```
+
 ## 6.6 Enumerating Properties
 
 ### 6.6.1 Property Enumeration Order