Get Variable from ASP.NET

Hello all,
i have a variable named "_var" defined on the ScriptSide
when i write a script in the HTML page the _variable imports the value and everything is working great.

Code:

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var _variable = "<%=_var %>";
        alert(_variable);

---

when i put those lines in a external JS file ("JSript.js") and import the file to the HTML page

Code:

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<script src="JScript.js" type="text/javascript"></script>

---

the _var isn't reckognized, and _variable returns the string itself insted of the value.

What am i doing wrong?