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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 _format_list(self, data):
 """ Format a list to use in javascript """

        dataset = "["
        i = 0
        for el in data:
            if pd.isnull(el):
                dataset += "null"
            else:
                dtype = type(data[i])
                if dtype == int or dtype == float:
                    dataset += str(el)
                else:
                    dataset += '"' + el + '"'
            if i < len(data) - 1:
                dataset += ', '
        dataset += "]"
        return dataset