deepmerge.js

const deepmerge = (h, ...args) => {
    const gettype = e => Object.prototype.toString.call(e).replace(/.*\b(\w+)./, '$1');
    
    const o = {};
    
    Object.assign(h, {
        "Array": (a, b) => [...a, ...b],
        "Object": (a, b) => deepmerge(h, a, b),
        "Set": (a, b) => new Set([...a, ...b]),
        "Map": (a, b) => {
            const o = new Map([...a]);
            for(let [k,v] of [...b]) {
                const vt = gettype(v);
                o.set(k, a.has(k) && vt in h ? h[vt](a.get(k), v) : v);
            }
            return o;
        }
    }, h);
    
    const p = new Proxy(o, {
        set: (o, k, b) => {
            const a = o[k];
            const at = gettype(a);
            return (o[k] = (at in h && k in o ? h[at](a, b) : b), true);
        }
    });
    
    Object.assign(p, ...args);
    
    return o;
};