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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def eval_js(self, expr):
 """Evaluate a Javascript expression."""

        if not self.is_built():
            self._pending_js_eval.append(expr)
            return
        logger.log(5, "Evaluate Javascript: `%s`.", expr)
        out = self.page().mainFrame().evaluateJavaScript(expr)
        return _to_py(out)