CODEWITHSUNDEEP
javascript
user2751079
user2751079

Reputation: 41

How can I check a variable is local or global in JavaScript?

When executing a JavaScript function, how can I decide the used variable is local or global?

Because I only want to record the modification to the global variable.

<script>
   var a;
   a =4;

   function foo(){
       var a =3;
   }()

</script>

when executing the above code, I only want to record the a=4, not a=3;

Upvotes: 4

Views: 1761

Answers (3)

Groove
Groove

Reputation: 121

I was looking for the same thing. When I couldn't find an answer, I ended up coming up with this:

<script>
    var a;
    a = {prop1: 'global object'};

    (function foo() {

        var a = {prop1: 'local object'};

        var isLocal = true;
        if (window.hasOwnProperty('a')) {
            if(Object.is(a, window['a'])) {
                isLocal = false;
            }
        }
    })();

</script>

If variable a is a primitive value, then Object.is() will always return true, so you'd need some other way to deal with them.

Upvotes: 0

Brad Christie
Brad Christie

Reputation: 101604

<script>
  var a;
  a = 4;
  function foo(){
    // version 1:
    if (window.hasOwnProperty('a')){
      // "global" a exists
    }
    // version 2:
    if (typeof window.a !== 'undefined'){
      // "global" a exists and is defined
    }
  }();
</script>

Something like that?

Upvotes: 7

RobG
RobG

Reputation: 147393

Global variables are added as properties of the global object, which is window in a browser. To test if an object has a property or not, use the in operator:

// In global scope
var bar;

function foo() {
  if (bar in window) {
    // bar is a property of window
  } else {
    // bar isn't a property of window
  }
}

For more general code, the environment may not have a window object, so:

// In global scope
var global = this;

function foo() {
  if (bar in global) {
    // bar is a property of the global object
  } else {
    // bar isn't a property of the global object
  }
}

Beware of typeof tests. They only tell you the type of the variable's value and return undefined if either the property doesn't exist or its value has been set to undefined.

Upvotes: 3

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