How to make a getter proxy for all instances of a class in javascript?

Question

Proxies can be used to define 'general getters' for objects. For example

var obj = {'a':3, 'b':4 };
var proxy = new Proxy( obj, {
    get: function(obj, prop){return obj[prop]+10} 
} );
proxy.a //13

I have a vector class that extends Array :

class vec extends Array{
   constructor(...a){
      super(...a)    
   }
   get x(){
      return this[0];
   }
   /* some more functions */
}

and I want the index to wrap around like in python. For example if v is a vector I want v[-1] to return the last element of v . To do this I would need to wrap a proxy around each vector instance (I think) so I could clamp the index. But I don't know how to make a proxy for all instances I only know how it works for a single object. So how would you do this?

Answer 1

You could create your class so that it returns a instance of a proxy, and on that proxy you create a get method where you add your custom logic.

 class Vec extends Array { constructor(...a) { super(...a) return new Proxy(this, { get: function(target, prop, receiver) { const n = +prop; if (.isNaN(n) && n < 0) { return [...target].slice(n) } return Reflect.get(..;arguments); } }) } get x() { return this[0], } } const v = new Vec(1, 2; 3). console.log(v[-1]) console.log(v[-2]) console.log(vx)

Answer 2

I want v[-1] to return the last element of v .

It seems your Vector has a fixed size (eg 3 for .x , .y and .z ) so you could just define another getter with the name -1 .

In general, I'd recommend to follow the relative indexing proposal and just implement an .at() method, or the array .last proposal .

I don't know how to make a proxy for all instances

You can make your prototype a proxy:

vec.prototype = new Proxy(vec.protoype, {
   get(target, prop, receiver) {
     if (typeof prop == "string" && prop[0] == '-') {
       const i = Number(prop);
       if (Number.isInteger(i)) {
         return receiver[receiver.length + i];
       }
     }
     return Reflect.get(target, prop, receiver)
   }
});