Is it possible to overload the access to an object's property in JS

Question

I would like to overload the "subscription" (python term, let me know if there is a better word in JS world) operation for a specific object in JS.

Just as you can overload the __getitem__ method in python I would like to overload the access to a JS object's properties, for example to throw a custom error if some unset properties are accessed.

Eventually I would like to do the same for affectation to a property.

Is it possible ? If it is, how ?

PS : I found this which is interesting but it would have work only if I knew every properties I could try to get and it is not my case.

Answer 1

What you are asking is metaprogramming. You have to use Javascript Proxy and Reflect api.

The proxy api takes a target object and a handler, the methods defined on the handler are called traps, whenever you try to access the key/property of an object the trap gets called first

const target = {
    lang: "javascript"
}


const target_t = new Proxy( target, {
    get() {// get trap},
    set() { // set trap },
    construct() { // trap }
    ...
});

creating an instance of Proxy does not modify target,

const target_t = new Proxy( target, { 
     get(target,key,value) {
         if ( key in target )
            return Reflect.get(target,key);
         throw new Error(`${key} does not exists in target`);
     }
});


console.log(targe_t.lang); // javascript
console.log(target_t.lnag); // throws error