url-search-params.proxy.ts

const handler: ProxyHandler<URLSearchParams> = {
    get(target, prop: string) {
        const values = target.getAll(prop);
        return values.length === 1 ? values[0] : values;
    },
    set(target, prop: string, value) {
        target.delete(prop);
        Array.isArray(value) ? value.forEach(v => target.append(prop, v)) : target.set(prop, value.toString())
        return true;
    },
    has(target, prop: string) {
        return target.has(prop)
    },
    deleteProperty(target, prop: string) {
        target.delete(prop);
        return true;
    },
    ownKeys(target) {
        return [...target.keys()]
    }
}

const params = new URLSearchParams('foo=1&bar=2');
const paramsProxy = new Proxy(params, handler) as any as { [key: string]: string | string[]; };

console.log(paramsProxy.foo) // '1'
console.log(paramsProxy.bar) // '2'

paramsProxy.foo = ['1', '3']
console.log(paramsProxy.foo) // ['1', '3']