(Deep?) copy Map in JavaScript

Question

How can I deep copy a map object to another map ? I'm trying to use ES6 method but this return Map(0){} empty object. My first try was send a map object to function and assign this to a new value in this class. But on next() method I'm removing one property from this map and this change map in unit test file

creatureTurnQueue.js

    export default class CreatureTurnQueue {
    constructor() {
        this.creatureMap = new Map();
        this.creatureArray = [];
        this.observersArray = [];
    }
    initQueue(list = {}) {
        this.creatureMap = Object.assign({}, list)
        this.creatureMap = list //<=old method

        list.forEach(val => this.creatureArray.push(val));
    }
    getActiveCreature() {
        let [first] = this.creatureArray.filter(el => el);
        return first;
    }
    next(list = {}) {
        this.creatureMap.delete(this.creatureMap.keys().next().value);
        if (this.creatureMap.size == 0) {

            this.notifyObserver();
            this.initQueue(list);
            return true;
        }
    }
    addObserver(_observer) {
        this.observersArray.push(_observer)
    }
    removeObserver(_observer) {
        this.observersArray.pull(_observer)
    }
    notifyObserver() {
        this.observersArray.forEach(item => item.resetCounterAttack())
    }
}

creatureTurnQueueTest.js

import Creature from "../creature.js";
import CreatureTurnQueue from "../creatureTurnQueue.js";
import Point from './../point';

export default class CreatureTurnQueueTest {
    queueShoulChangeActiveCreature() {
        let creatureTurnQueue = new CreatureTurnQueue();

        let creture1 = new Creature("aaaa", 1, 1, 1, 1);
        let creture2 = new Creature("bbbb", 1, 1, 1, 1);
        let creture3 = new Creature("cccc", 1, 1, 1, 1);

        let point1 = new Point(1, 0)
        let point2 = new Point(2, 0)
        let point3 = new Point(3, 0)

        let creatureMap = new Map();
        creatureMap.set(point1, creture1);
        creatureMap.set(point2, creture2);
        creatureMap.set(point3, creture3);

        creatureTurnQueue.initQueue(creatureMap);
        console.log("~ creatureMap", creatureMap) <= map have 2 elements
        creatureMap.forEach(item => { <= creatureMap return 2 elements becouse this one value is removed
            if (item !== creatureTurnQueue.getActiveCreature()) {
                console.log("~ item", item)
                console.log("~ creatureTurnQueue.getActiveCreature()", creatureTurnQueue.getActiveCreature())
                throw `Exception: => Kolejka nie dziala poprawnie zwracana aktywna creatura jest inna`;
            }
            if (creatureTurnQueue.next(creatureMap)) {
                throw `Exception: => Kolejka nie dziala poprawnie w momecie wywolania funkcji next()`;
            }
        });
    }
}

The rest of a classes point.js

export default class Point {
    constructor(_x, _y) {
        this.x = _x;
        this.y = _y;
    }
}

creature.js

import CreatureStatistics from "./creatureStatistics.js";

export default class Creature {
    constructor(_name, _attack, _armor, _maxHp, _moveRange) {
        this.stats = this.createCreature(_name, _attack, _armor, _maxHp, _moveRange);
        this.stats.currentHp = this.stats.maxHp;
        this.stats.wasCounterAttack = false;
    }
    createCreature(_name, _attack, _armor, _maxHp, _moveRange) {
        return new CreatureStatistics(
            _name || "Smok",
            _attack || 1,
            _armor || 1,
            _maxHp || 100,
            _moveRange || 10
        );
    }
    setDefaultStats() {
        // this.stats.wasCounterAttack == true ? this.stats.wasCounterAttack = false : this.stats.wasCounterAttack = true
        this.stats.currentHp = this.stats.currentHp != undefined ? this.stats.currentHp : this.stats.maxHp;
    }
    // popraw counter atack
    attack(_defender) {
        _defender.setDefaultStats();
        this.setDefaultStats();

        if (_defender.isAlive()) {
            _defender.stats.currentHp = this.calculateDamage(_defender);
            if (_defender.isAlive() && !_defender.stats.wasCounterAttack) {
                _defender.stats.wasCounterAttack = true;
                this.stats.currentHp = _defender.calculateDamage(this);
            }
        }
    }
    calculateDamage(attackedCreature) {
        return attackedCreature.stats.currentHp - this.stats.getAttack() + attackedCreature.stats.getArmor() > attackedCreature.stats.getMaxHp()
            ? attackedCreature.stats.currentHp
            : attackedCreature.stats.currentHp - this.stats.getAttack() + attackedCreature.stats.getArmor();
    }
    isAlive() {
        if (this.stats.currentHp > 0) {
            return true;
        }
    }
    getCurrentHp() {
        return this.stats.currentHp;
    }
    resetCounterAttack() {
        this.stats.wasCounterAttack = false;
    }
    canCounterAttack() {
        return !this.stats.wasCounterAttack
    }

}

creatureStatistics.js

export default class CreatureStatistics {
    constructor(_name, _attack, _armor, _maxHp, _moveRange) {
        this.name = _name;
        this.attack = _attack;
        this.armor = _armor;
        this.maxHp = _maxHp;
        this.moveRange = _moveRange;
    }
    getName() {
        return this.name;
    }
    getAttack() {
        return this.attack;
    }
    getArmor() {
        return this.armor;
    }
    getMaxHp() {
        return this.maxHp;
    }
    getMoveRange() {
        return this.moveRange;
    }
}

You don't want to deep copy your map. You just need to copy it at all. — Bergi, Dec 01 '20 at 13:50

score 2 · Accepted Answer · answered Dec 01 '20 at 10:37

The map was in fact a reference of the creatureMap from unit test script. Modifying the map inside your class will modify the creatureMap in unit test. You can try to create a new Map and copy over all values:

// Start class CreateTurnQueue
initQueue(list = {}) {
   
   const newMap = new Map();
   
   // Both Object and Map has entries method although the order is different
   const iterator = list.entries();
   
   for(const item of iterator) {
      const [point, creature] = item;
      newMap.set(point, creature);
      this.creatureArray.push(creature);
   }
   this.creatureMap = newMap;
   
}
// End class CreateTurnQueue

Now you don't have a reference to the creatureMap from the unit test script and modify this.creatureMap will not affect the one you passed to the initQueue method as argument.

No need to make it so complicated, though: `this.creatureMap = new Map(list); this.creatureArray = Array.from(list.values());` — Bergi, Dec 01 '20 at 11:20

(Deep?) copy Map in JavaScript

1 Answers1