  We present observations of ZTF18abfcmjw (SN2019dge), a helium-rich supernova
with a fast-evolving light curve indicating an extremely low ejecta mass
($\approx 0.3\,M_\odot$) and low kinetic energy ($\approx 1.2\times
10^{50}\,{\rm erg}$). Early-time (<4 d after explosion) photometry reveal
evidence of shock cooling from an extended helium-rich envelope of
$\sim0.1\,M_\odot$ located at $\sim 3\times 10^{12}\,{\rm cm}$ from the
progenitor. Early-time He II line emission and subsequent spectra show
signatures of interaction with helium-rich circumstellar material, which
extends from $\gtrsim 5\times 10^{13}\,{\rm cm}$ to $\gtrsim 2\times
10^{16}\,{\rm cm}$. We interpret SN2019dge as a helium-rich supernova from an
ultra-stripped progenitor, which originates from a close binary system
consisting of a mass-losing helium star and a low-mass main sequence star or a
compact object (i.e., a white dwarf, a neutron star, or a black hole). We infer
that the local volumetric birth rate of 19dge-like ultra-stripped SNe is in the
range of 1400--8200$\,{\rm Gpc^{-3}\, yr^{-1}}$ (i.e., 2--12% of core-collapse
supernova rate). This can be compared to the observed coalescence rate of
compact neutron star binaries that are not formed by dynamical capture.
