Javascript - catch access to property of object [duplicate]

Question

Is it possible to capture when a (any) property of an object is accessed, or attempting to be accessed?

Example:

I have created custom object Foo

var Foo = (function(){     var self = {};     //... set a few properties     return self; })(); 

Then there is some action against Foo - someone tries to access property bar

Foo.bar 

Is there way (prototype, perhaps) to capture this? bar may be undefined on Foo. I could suffice with capturing any attempted access to undefined properties.

For instance, if bar is undefined on Foo, and Foo.bar is attempted, something like:

Foo.prototype.undefined = function(){     var name = this.name; //name of property they attempted to access (bar)     Foo[name] = function(){         //...do something     };     return Foo[name]; } 

But functional, unlike my example.

Concept

Foo.* = function(){ } 

Background

If I have a custom function, I can listen for every time this function is called (see below). Just wondering if it's possible with property access.

Foo = function(){}; Foo.prototype.call = function(thisArg){     console.log(this, thisArg);     return this; } 

Answer 1

Yes, this is possible in ES2015+, using the Proxy. It's not possible in ES5 and earlier, not even with polyfills.

It took me a while, but I finally found my previous answer to this question. See that answer for all the details on proxies and such.

Here's the proxy example from that answer:

const obj = new Proxy({}, {     get: function(target, name, receiver) {         if (!(name in target)) {             console.log("Getting non-existant property '" + name + "'");             return undefined;         }         return Reflect.get(target, name, receiver);     },     set: function(target, name, value, receiver) {         if (!(name in target)) {             console.log("Setting non-existant property '" + name + "', initial value: " + value);         }         return Reflect.set(target, name, value, receiver);     } });  console.log("[before] obj.foo = " + obj.foo); obj.foo = "bar"; console.log("[after] obj.foo = " + obj.foo); obj.foo = "baz"; console.log("[after] obj.foo = " + obj.foo); 

Live Copy:

"use strict";    const obj = new Proxy({}, {      get: function(target, name, receiver) {          if (!(name in target)) {              console.log("Getting non-existant property '" + name + "'");              return undefined;          }          return Reflect.get(target, name, receiver);      },      set: function(target, name, value, receiver) {          if (!(name in target)) {              console.log("Setting non-existant property '" + name + "', initial value: " + value);          }          return Reflect.set(target, name, value, receiver);      }  });    console.log("[before] obj.foo = " + obj.foo);  obj.foo = "bar";  console.log("[after] obj.foo = " + obj.foo);  obj.foo = "baz";  console.log("[after] obj.foo = " + obj.foo);

When run, that outputs:

Getting non-existant property 'foo' [before] obj.foo = undefined Setting non-existant property 'foo', initial value: bar [after] obj.foo = bar [after] obj.foo = baz