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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 install_js():
    ''' Copy built BokehJS files into the Python source tree.

    Returns:
        None

    '''
    target_jsdir = join(SERVER, 'static', 'js')
    target_cssdir = join(SERVER, 'static', 'css')
    target_tslibdir = join(SERVER, 'static', 'lib')

    STATIC_ASSETS = [
        join(JS,  'bokeh.js'),
        join(JS,  'bokeh.min.js'),
        join(CSS, 'bokeh.css'),
        join(CSS, 'bokeh.min.css'),
    ]
    if not all(exists(a) for a in STATIC_ASSETS):
        print(BOKEHJS_INSTALL_FAIL)
        sys.exit(1)

    if exists(target_jsdir):
        shutil.rmtree(target_jsdir)
    shutil.copytree(JS, target_jsdir)

    if exists(target_cssdir):
        shutil.rmtree(target_cssdir)
    shutil.copytree(CSS, target_cssdir)

    if exists(target_tslibdir):
        shutil.rmtree(target_tslibdir)
    if exists(TSLIB):
        # keep in sync with bokehjs/src/compiler/compile.ts
        lib = {
            "lib.es5.d.ts",
            "lib.dom.d.ts",
            "lib.es2015.core.d.ts",
            "lib.es2015.promise.d.ts",
            "lib.es2015.symbol.d.ts",
            "lib.es2015.iterable.d.ts",
        }
        shutil.copytree(TSLIB, target_tslibdir, ignore=lambda _, files: [ f for f in files if f not in lib ])