Javascript from ASP.NET C#

I would like to do something like the following:
_btnImport.Click += new EventHandler(this._btnImport_Click);
private void _btnImport_Click(object sender, EventArgs e)
{
   if (ValidFile()
       if (confirm('Are you sure you would like to import this file?') // this is a Javascript call
            ImportFile();
}
I'm sure this is possible to do but I'm at a loss. Any suggestions or links to other threads would be greatly appreciated. TIA
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