CODEWITHSUNDEEP
javascript
vanowm
vanowm

Reputation: 10221

Cannot redefine property when trying change property of a global variable

How can we set custom getter/setter on already existing global variable? A straightforward approach fails:

var myVar;

{
  let _myVar = myVar;
  delete myVar;
  Object.defineProperty(window, "myVar", {
    get()
    {
      return _myVar;
    },
    set(val)
    {
      _myVar = val;
    }
  });
}

Upvotes: 0

Views: 74

Answers (1)

Nick Vu
Nick Vu

Reputation: 15540

var myVar;

According to this document:

Any property declared with var cannot be deleted from the global scope or from a function's scope.

Therefore, delete myVar won't get applied, and that's why you cannot set a value again for myVar with defineProperty.

For a simple test, you can try to log myVar value after delete myVar.

var myVar = 5; //set a value for `myVar`

{
  let _myVar = myVar;
  delete myVar; //delete `myVar`
  console.log(myVar); //it's still 5
  Object.defineProperty(window, "myVar", {
    get()
    {
      return _myVar;
    },
    set(val)
    {
      _myVar = val;
    }
  });
}

For a potential solution, you can explicitly set a property to window object which is considered as a global variable.

window.myVar = 5; //set a value for `window.myVar`

{
  let _myVar = window.myVar;
  delete window.myVar;
  console.log(window.myVar); //`undefined` as expected
  Object.defineProperty(window, "myVar", {
    get() {
      return _myVar;
    },
    set(val) {
      _myVar = val;
    }
  });
}

Just one side note that in terms of global variable usage, var and window are not different. You can declare window.myVar, but you can use myVar (without window) directly.

window.myVar = 5; //set a value for `window.myVar`

{
  let _myVar = myVar;
  delete window.myVar;
  console.log(window.myVar); //`undefined` as expected
  Object.defineProperty(window, "myVar", {
    get() {
      return _myVar;
    },
    set(val) {
      _myVar = val;
    }
  });
  console.log(myVar); //set it again with `defineProperty`
}

Upvotes: 1

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