MapBuilder

map-builder.ts

interface IMapValueFactory<K, T> {
    construct: (key: K) => T;
}

interface IMapBuilder<K, T> {
    maybeConstruct: (key: K) => void;
    get: (
        ((key: K, maybeConstruct?: boolean) => T | undefined) &
        ((key: K, maybeConstruct: true) => T)
    );
}

class MapBuilder<K, T> extends Map<K, T> implements IMapBuilder<K, T> {
    private valueFactory: IMapValueFactory<K, T>;
    constructor(valueFactory: IMapValueFactory<K, T>) {
        super();
        this.valueFactory = valueFactory;
    }
    public maybeConstruct(key: K) {
        if (!this.has(key)) {
            this.set(key, this.valueFactory.construct(key));
        }
    }
    public get(key: K, maybeConstruct: boolean = false) {
        if (maybeConstruct) this.maybeConstruct(key);
        return super.get(key);
    }
}