push3-1.js

function spyFactory(array, prepushHandler, postpushHandler) {
    const prepush = functionify(prepushHandler);
    const postpush = functionify(postpushHandler);
    const interceptor = {
        get: function (obj, prop) {
            if (prop !== 'push') { return obj[prop]; }

            return function (...args) {
                prepush(args);
                Array.prototype.push.apply(this, args);
                postpush(args);
            }
        }
    }

    function functionify(maybeNotAFunction) {
        return typeof maybeNotAFunction === 'function' ? maybeNotAFunction : () => null;
    }

    return new Proxy(array, interceptor);
}

const spyedArray = spyFactory([77, 5], (arg) => { console.log(`prepush called with ${arg}`) });
spyedArray.push('4' );
console.log(spyedArray[2]);