Demonstration of how Object.assign, object mutation, and proxy binding play together in JS

proxy-assign-mem-share.js

const a = {
    a: 100
}

const c = {
    c: 300
}

function get_c_proxy(mainObj) {
    return new Proxy(c, {
        get: function (object, name) {
            return object[name] + mainObj.b;
        }
    })
}

/**
 * Because we're passing "a" to Object.assign,
 * `b` will be mutated onto the
 * existing variable memory address
 *
 * Then, later, when we pass that object to `get_c_proxy`, it
 * will be able to access that vairable
 */
Object.assign(a, {
    b: 200
}, get_c_proxy(a));

console.log(c.c); // Logs "300". Reading original object
console.log(a.c); // Logs "500". Reading from newly assigned data

// Keep in mind that `Object.assign` only works by iterating through props and assigning the values.
// This means they're no longer trapped via the proxy and won't update future proxied object values

c.c = 0;
console.log(c.c); // 0
console.log(a.c); // 500. Was not updated