  Warm Dark Matter (WDM) particles with masses ($\sim$ kilo electronvolt) offer
an attractive solution to the small-scale issues faced by the Cold Dark Matter
(CDM) paradigm. The delay of structure formation in WDM models and the
associated dearth of low-mass systems at high-redshifts makes this an ideal
time to revisit WDM constraints in light of the unprecedented data-sets from
the James Webb Space Telescope (JWST). Developing a phenomenological model
based on the halo mass functions in CDM and WDM models, we calculate
high-redshift ($z \gt 6$) the stellar mass functions (SMF) and the associated
stellar mass density (SMD) and the maximum stellar mass allowed in a given
volume. We find that: (i) WDM as light as 1.5 keV is already disfavoured by the
low-mass end of the SMF (stellar mass $M_* \sim 10^7 \rm{M_\odot}$) although
caution must be exerted given the impact of lensing uncertainties; (ii) 1.5 keV
WDM models predict SMD values that show a steep decrease from $10^{8.8}$ to
$10^{2} ~{\rm M_\odot ~cMpc^{-3}}$ from $z \sim 4$ to 17 for $M_* \gt 10^8
\rm{M_\odot}$; (iii) the 1.5 keV WDM model predicts a sharp and earlier cut-off
in the maximum stellar masses for a given number density (or volume) as
compared to CDM or heavier WDM models. For example, with a number density of
$10^{-3} \rm {cMpc^{-3}}$, 1.5 (3) KeV WDM models do not predict bound objects
at $z \gt 12$ (18). Forthcoming JWST observations of multiple blank fields can
therefore be used as a strong probe of WDM at an epoch inaccessible by other
means.
