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Weird behavior in referring to global variable. Is this a bug in javascript? Surely it isn't!

Answer 1

+1 A:

The scope of x is the ~~block~~ function in which it is declared... although I believe scope in JavaScript can be a bit tricky sometimes. In C# this would be a compile-time error - it would be trying to use the local variable before its declaration.

Whatever the reason, I'd try to avoid doing it simply for the sake of readability.

Jon Skeet 2009-02-25 12:10:46

Answer 2

+3 A:

The second var-declaration is interfering with the first. You are actually referring to the as-of-yet-undeclared local x. However, to quote javascript guru Douglas Crockford:

JavaScript's biggest problem is its dependence on global variables, particularly implied global variables. If a variable is not explicitly declared (usually with the var statement), then JavaScript assumes that the variable was global. This can mask misspelled names and other problems.

http://www.jslint.com/lint.html

So the recommendation is to avoid using global variables as much as possible.

Magnar 2009-02-25 12:11:38

Answer 3

+5 A:

The ECMAScript Specification says in section 12.2:

If the variable statement occurs inside a FunctionDeclaration, the variables are defined with function-local scope in that function, as described in s10.1.3. Otherwise, they are defined with global scope (that is, they are created as members of the global object, as described in 10.1.3) using property attributes { DontDelete}. Variables are created when the execution scope is entered. A Block does not define a new execution scope. Only Program and FunctionDeclaration produce a new scope. Variables are initialised to undefined when created. A variable with an Initialiser is assigned the value of its AssignmentExpression when the VariableStatement is executed, not when the variable is created.

So it's not a bug - the local variable is created when the function is entered.

Greg 2009-02-25 12:14:44

The ECMAScript Specification's excerpt that you have provided explains it very clearly. But, got to admit that the mechanism creates confusing situation like the one discussed in the question!

Chandan . 2009-02-25 12:52:25

Answer 4

A:

I changed "var x" to "x". It is working now.

<html>
<body>
<script language="javascript">  
var x = 5; //globally declaredfunction 

function showX() {     
  alert("x="+x); //trying to display global value    

/*var */   x=10; //trying to create and initialize a local x
  }
  </script>

  <input type = "button" value="Show X" onclick="showX()" /> 

  </body>
  </html>

TheMachineCharmer 2009-02-25 12:24:59

You're not intializing a local variable x. You're overwriting the global variable.

Magnar 2009-02-25 12:26:37

yeah. the whole purpose is defeated. by commenting 'var' it changes the original problem altogether!

Chandan . 2009-02-25 12:39:53

Ooops I misunderstud the problem . Sorry!!!

TheMachineCharmer 2009-02-25 12:45:33

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Weird behavior in referring to global variable. Is this a bug in javascript? Surely it isn't!

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