  We investigate the weak cosmic censorship conjecture in Kerr-(anti-)de Sitter
black holes under the scattering of a scalar field. We test the conjecture in
terms of whether the black hole can exceed the extremal condition with respect
to its change caused by the energy and angular momentum fluxes of the scalar
field. Without imposing the laws of thermodynamics, we prove that the
conjecture is valid in all the initial states of the black hole (non-extremal,
near-extremal, and extremal black holes). The validity in the case of the
near-extremal black hole is different from the results of similar tests
conducted by adding a particle because the fluxes represent the energy and
angular momentum transferred to the black hole during the time interval not
included in the tests involving the particle. Using the time interval, we show
that the angular velocity of the black hole with the scalar field of a constant
state takes a long time for saturation to the frequency of the scalar field.
