CODEWITHSUNDEEP
javascriptjquery
Michael Samuel
Michael Samuel

Reputation: 3920

Variable loses value on if statement in javascript

I have this simple HTML layout as a test page:

<html>
<head>
  <script src="http://code.jquery.com/jquery-latest.min.js" type="text/javascript">
  </script>

  <script>var user = 'test';</script>

  <script>
  $(document).ready(function () {
       if(!user) {
          var user = '0';
       }
       alert(user);
  });
  </script>
</head>
</html>

For some reason when checking the value of !user using the if condition, the condition is successful and the variable user gets the new value of 0 although I defined it previously. This has been driving me crazy for sometime now. Is there something wrong with the above code?

Upvotes: 1

Views: 1727

Answers (2)

Rahul Singh
Rahul Singh

Reputation: 21795

When DOM is loading variable 'user' is undefined. And in JavaScript (!undefined) is true, thus you are getting zero as output.

Upvotes: 1

Pointy
Pointy

Reputation: 413757

You've redeclared "user" as a local variable in the function. Variable definitions in functions are always treated as if they occur at the very start of the function, meaning your code is equivalent to

  $(document).ready(function () {
       var user;
       if(!user) {
          user = '0';
       }
       alert(user);
  });

At the if statement, the value of "user" will be undefined. The !user test succeeds, and so your code proceeds to set the variable to '0'.

Upvotes: 4

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