Resolve promise on array push - javascript

The problems with your attempt:

• you invoke .push on the original array, not the proxied one. Where you create the proxy, it is returned to no-one: any reference to it is lost (and will be garbage collected).
• The code following after the line with await will execute asynchronously, so after all of your push calls have already executed. That means that console.log will execute when the array already has two elements. Promises are thus not the right tool for what you want, as the resolution of a promise can only be acted upon when all other synchronous code has run to completion. To get notifications during the execution synchronously, you need a synchronous solution, while promises are based on asynchronous execution.

Just to complete the answer, I provide here a simple synchronous callback solution:

function observed(array, cb) {
    return new Proxy(array, {
        set(array, key, val) {
            array[key] = val;
            if (!isNaN(key)) cb(); // now it is synchronous
            return true;
        }
    });
}

let a = observed([], () =>
    console.log(new Date().toLocaleTimeString(),"Blimey Guv'nor:", `${a.pop()}`)
);

a.push('ʕ·͡ᴥ·ʔ');
a.push('¯\(°_o)/¯ ');

As noted before: promises are not the right tool when you need synchronous code execution.

When each push is executed asynchronously

You can use promises, if you are sure that each push happens in a separate task, where the promise job queue is processed in between every pair of push calls.

For instance, if you make each push call as part of an input event handler, or as the callback for a setTimeout timer, then it is possible:

function observed(array) {
    let resolve = () => null; // dummy
    let proxy = new Proxy(array, {
        set(array, key, val) {
            array[key] = val;
            if (!isNaN(key)) resolve();
            return true;
        }
    });
    proxy.observe = () => new Promise(r => resolve = r);
    return proxy;
}


let a = observed([]);
(async () => {
    while (true) {
        await a.observe();
        console.log(new Date().toLocaleTimeString(),"Blimey Guv'nor:",`${a.pop()}`);
    }
})();

setTimeout(() => a.push('ʕ·͡ᴥ·ʔ'), 100);
setTimeout(() => a.push('¯\(°_o)/¯ '), 100);