How to Extends / Alias / Overwrite existing class in MZ/ ES6 [Tutorial]

/*
* Do these 2 additional things when using ES6 class inheritance aliasing
* without directly typing prototypes:
* 1. Add the following code right below a new class inheriting another one:
*    - ExtendedClassAlias.inherit(Klass);
*    Where Klass is the new class inheriting another one
* 2. Add the following code right below extending an existing class as a way
*    to alias its methods:
*    - ExtendedClassAlias.updateClass(Klass);
*    Where Klass is the existing class being extended as a way to alias its
*    methods
* Right now it doesn't work well with inheriting static functions in classes,
* so those in children classes should use ParentClass.staticFunc.call(this)
* instead of super.staticFunc()
*/

var GA_proto = Game_Actor.prototype;
var old_setup = GA_proto.setup;
GA_proto.setup = function() {...}

JsonEx._encode = function(value, depth) {
    // [Note] The handling code for circular references in certain versions of
    //   MV has been removed because it was too complicated and expensive.
    if (depth >= this.maxDepth) {
        throw new Error("Object too deep");
    }
    const type = Object.prototype.toString.call(value);
    if (type === "[object Object]" || type === "[object Array]") {
        const constructorName = value.constructor.name;
        if (constructorName !== "Object" && constructorName !== "Array") {
            value["@"] = constructorName;
        }
        for (const key of Object.keys(value)) {
            value[key] = this._encode(value[key], depth + 1);
        }
    }
    return value;
};

JsonEx._decode = function(value) {
    const type = Object.prototype.toString.call(value);
    if (type === "[object Object]" || type === "[object Array]") {
        if (value["@"]) {
            const constructor = window[value["@"]];
            if (constructor) {
                Object.setPrototypeOf(value, constructor.prototype);
            }
        }
        for (const key of Object.keys(value)) {
            value[key] = this._decode(value[key]);
        }
    }
    return value;
};

let i = 0
[0, 1, 2, 3, 4, 5, 6].forEach(item => console.log(item))

let i = 0;
[0, 1, 2, 3, 4, 5, 6].forEach(item => console.log(item))

let a = 1
let b = a
(a + b).toString() // Will give an error

let a = 1
-a + a === 0 ? console.log(0) : console.log(2)
// Is it 0? NOPE without semicolons it will be 2!
let a = 1;
-a + a === 0 ? console.log(0) : console.log(2) // Now it will be 0

let i = 0 let j = 1 ... // Errors
let i = 0; let j = 1; ... // No errors

let obj = function {} obj.prototype.x = function () {} let y = 1 function b() {} let a = '' // Several errors
let obj = function () {}; obj.prototype.x = function () {}; let y = 1; function b() {} let a = ''; // No errors...
// Notice that for example function b() {} doesn't need semicolons even in single line mode