For blog.

ChainOfMpCost.js

(function() {
    'use strict';

////=============================================================================
//// Game_Actor
////  Override for Chain MP cost.
////=============================================================================
    Game_Actor.prototype.canPaySkillCost = function(skill) {
        return this._tp >= this.skillTpCost(skill) && ChainManager.getPartyMP() >= this.skillMpCost(skill);
    };

    Game_Actor.prototype.paySkillCost = function(skill) {
        this._mp -= this.skillMpCost(skill);
        this._tp -= this.skillTpCost(skill);

        ChainManager.tryExecuteChainMpCost(this);
    };

    Game_Actor.prototype.chainSkillMpCost = function(cost) {
        this._mp -= cost;
        ChainManager.tryExecuteChainMpCost(this);
    };

////=============================================================================
//// BattleManager
////  Override for Chain MP cost.
////=============================================================================
    const _BattleManager_startBattle = BattleManager.startBattle;
    BattleManager.startBattle = function() {
        _BattleManager_startBattle.call(this);
        ChainManager.initializeBattlers();
    };

////=============================================================================
//// ChainManager
////  Manage Chain MP cost.
////=============================================================================
    class ChainManager {
        static initializeBattlers() {
            this._battlers = $gameParty.battleMembers();
            console.log("initializeBattlers");
        }

        /**
         * @return {Array}
         */
        static get battlers() {
            return this._battlers;
        }

        static tryExecuteChainMpCost(currentBattler) {
            const nextBattler = ChainManager.getNextBattler(currentBattler);
            console.log(nextBattler);
            if(nextBattler && currentBattler._mp <= 0) {
                nextBattler.chainSkillMpCost(Math.abs(currentBattler._mp));
                currentBattler._mp = 0;
            }
        }

        static getPartyMP() {
            let total = 0;
            this.battlers.forEach(function(value, index) {
                total += value.mp;
            });
            return total;
        }

        static getNextBattler(currentBattler) {
            let nextIndex;
            this.battlers.some(function(battler, index) {
                if(battler === currentBattler) {
                    nextIndex = (index + 1)%this.battlers.length;
                    return true;
                }
            }, this);
            return this.battlers[nextIndex];
        }
    }

})();