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home > news > java technology > eval javascript in a global context

Eval JavaScript in a global context

Even though it's considered bad practice, it's often handy to eval code in JavaScript.  And in my case, it was simply necessary, since the JSF specification requires eval of scripts. And it's also necessary to execute those evaluated scripts in the global scope. It's not as easy as it first looks. For our first naive implementation, we'd simply used eval(src) in our first pass at the implementation. This is utterly wrong, and to understand why, you'll need to understand scopes. JavaScript has what you can think of as two different scopes - function scope, where you're executing something in the context of a function, and global scope, where you're executing something in a global context - for instance, if I say var j = 1; within a function's scope, then the variable j is set to 1 within that function. If I say the same expression, var j = 1 within the global scope, then j is set to 1 everywhere in the program - in every function, provided that that function doesn't define a j variable in its local scope. In browsers, the global context is window - this is the default object that everything gets hung off of if you don't specify any other object. So, when we said eval(src), we were executing the src scripts within the local scope of the function where eval was called - that meant that I would be getting different results when variables were declared and set than wou...


Date: September, 08 2009
Url: http://www.java.net/blog/driscoll/archive/2009/09/08/eval-javascript-global-context


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