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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 list_js_files(dir):
 """ Generator for all JavaScript files in the directory, recursively True """

    for dirpath, dirnames, filenames in os.walk(dir):
        for filename in filenames:
            if is_js_file(filename):
                yield os.path.join(dirpath, filename)