  We introduce a generalized joint density of states (GJDOS), which
incorporates the coherent factor into the JDOS, to study quasiparticle
interference (QPI) in superconductors. The intimate relation between the
Fourier-transformed local density of states and GJDOS is revealed: they corre-
spond respectively to the real and imaginary parts of a generalized
impurity-response function, and particularly share the same angular factors and
singular boundaries, as seen from our approximate analytic results for d-wave
superconductors. Remarkably, our numerical GJDOS analysis agrees well with the
QPI patten of d-wave cuprates and s\pm-wave iron-based superconductors.
Moreover, we illustrate that the present GJDOS scenario can uncover the sign
features of the superconducting gap and thus can be used to explore the
possible pairing symmetry of the KxFe2-ySe2 superconductors.
