How can I detect when a global variable is set in javascript?

Question

In javascript, I can detect when a global variable foo is set (assuming that it is initially undefined) by using Object.defineProperty:

var foo_;
Object.defineProperty(window, 'foo', {
  get: function() {
    return foo_;
  },
  set: function(newFoo) {
    console.log('foo set to ' + newFoo);
    foo_ = newFoo;
  }
});

Is there a more elegant way to do this? One downside is that I cannot call Object.defineProperty on the same property twice. Overriding the entire property just to detect when it is defined seems a bit overkill.

Could I somehow use proxies? I feel like I would have to make a proxy that targets the window object though ... is that efficient?

I think we can check if it is available in window object like window[foo] — techie_28
– techie_28, Commented Aug 4, 2016 at 5:32
I want to detect the moment when the global variable is set and say log something at that time. — John Hoffman
– John Hoffman, Commented Aug 4, 2016 at 5:35
This feels like an async hack. If it is, don't do it. Use a Promise — castletheperson
– castletheperson, Commented Aug 4, 2016 at 5:37
If you use it for debugging - why does it matter if you cannot set it twice? If you're using it not for debugging - you better don't. — zerkms
– zerkms, Commented Aug 4, 2016 at 5:47
I'm detecting for debugging purposes, but another library has already called Object.defineProperty on that property. — John Hoffman
– John Hoffman, Commented Aug 4, 2016 at 5:49

Bergi · Accepted Answer · 2016-08-04 06:11:07Z

1

Is there a more elegant way to do this?

Not really. Maybe you could (should) hide _foo in a closure instead of creating another global variable.

One downside is that I cannot call Object.defineProperty on the same property twice. Overriding the entire property just to detect when it is defined seems a bit overkill.

The only problem is that you forgot to pass the configurable: true option.

Could I somehow use proxies?

No. You cannot replace the global object with a proxy. The only thing you could do is wrap all the code in a with (new Proxy(…)) { … } block.

edited Aug 4, 2016 at 6:11

answered Aug 4, 2016 at 6:04

Bergi

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Redu · Accepted Answer · 2016-08-04 05:56:39Z

-1

You might also do like

var _bar;
window.__defineGetter__("bar", function(){return this._bar});
window.__defineSetter__("bar", function(v){console.log("bar set to:",v); this._bar = v;});

answered Aug 4, 2016 at 5:56

Redu

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1 Comment

Bergi Over a year ago

Don't use deprecated and non-standard builtins.

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