a typescript map that generates default values if none exists

gistfile1.txt

/**
 * a map that generates default values if none exists
 */
export class MagicMap<K,V> extends Map<K,V> {
    constructor(private defVal: () => V){
        super();
    }
    has = (key:K) => true;
    get = (key:K): V => {
        if (!Map.prototype.has.call(this, key)){
            this.set(key, this.defVal());
        }
        return Map.prototype.get.call(this, key);
    }
}