Finished Chapter 3.8 Immutable Primitive and Mutable Objectives

notes/chap3_types_values_and_variables.md

@@ -536,6 +536,66 @@ Since ES2020, `globalThis` is used to refer to the global obj.
 
 ## 3.8 Immutable Primitive Values and Mutable Object References
 
+Summary: in JS, primitive values are immutable, object values are mutable
+
+Primitives are immutable:
+* Only new values can be created/returned
+* Can be compared by value:
+  * Two string/array are equal iif they have same lenght, and char at index is the same.
+```js
+let s = "hello";   // Start with some lowercase text
+s.toUpperCase();   // Returns "HELLO", but doesn't alter s
+s                  // => "hello": the original string has not changed
+```
+
+Objects are mutable:
+* Their values can change
+* Not compared by value. Two distinct/independent obj are not equal even if they have the same properties and values.
+* They are also called **reference types** to be distinguished from JS's primitive types.
+  * Two obj values are the same iff they **refer** to the same underlying object. e.g. `let b = a`
+
+Mutable obj
+```js
+let o = { x: 1 };  // Start with an object
+o.x = 2;           // Mutate it by changing the value of a property
+o.y = 3;           // Mutate it again by adding a new property
+
+let a = [1,2,3];   // Arrays are also mutable
+a[0] = 0;          // Change the value of an array element
+a[3] = 4;          // Add a new array element
+```
+
+Two distinct/independent obj
+```js
+let o = {x: 1}, p = {x: 1};  // Two objects with the same properties
+o === p                      // => false: distinct objects are never equal
+let a = [], b = [];          // Two distinct, empty arrays
+a === b                      // => false: distinct arrays are never equal
+```
+
+Obj are same if they refer the same obj:
+```js
+let a = [];   // The variable a refers to an empty array.
+let b = a;    // Now b refers to the same array.
+b[0] = 1;     // Mutate the array referred to by variable b.
+a[0]          // => 1: the change is also visible through variable a.
+a === b       // => true: a and b refer to the same object, so they are equal.
+```
+
+How to compare 2 distinct objs:
+* Compare their properties/elements one by one
+
+```js
+function equalArrays(a, b) {
+    if (a === b) return true;                // Identical arrays are equal
+    if (a.length !== b.length) return false; // Different-size arrays not equal
+    for(let i = 0; i < a.length; i++) {      // Loop through all elements
+        if (a[i] !== b[i]) return false;     // If any differ, arrays not equal
+    }
+    return true;                             // Otherwise they are equal
+}
+```
+
 ## 3.9 Type Conversions
 
 ### 3.9.1 Conversions and Equality