  We present a study of SN 2009js in NGC 918. Multi-band Kanata optical
photometry covering the first ~120 days show the source to be a Type IIP SN.
Reddening is dominated by that due to our Galaxy. One-year-post-explosion
photometry with the NTT, and a Subaru optical spectrum 16 days post-discovery,
both imply a good match with the well-studied subluminous SN 2005cs. The
plateau phase luminosity of SN 2009js and its plateau duration are more similar
to the intermediate luminosity IIP SN 2008in. Thus, SN 2009js shares
characteristics with both subluminous and intermediate luminosity SNe. Its
radioactive tail luminosity lies between SN 2005cs and SN 2008in, whereas its
quasi-bolometric luminosity decline from peak to plateau (quantified by a
newly-defined parameter Delta[logL] measuring adiabatic cooling following shock
breakout) is much smaller than both the others. We estimate the ejected mass of
56Ni to be low (~0.007 Msun). The SN explosion energy appears to have been
small, similar to SN 2005cs. SN 2009js is the first subluminous SN IIP to be
studied in the mid-infrared. It was serendipitously caught by Spitzer at very
early times. In addition, it was detected by WISE 105 days later with a
significant 4.6 micron flux excess above the photosphere. The infrared excess
luminosity relative to the photosphere is clearly smaller than that of SN
2004dj extensively studied in the mid-infrared. The excess may be tentatively
assigned to heated dust with mass ~3e-5 Msun, or to CO fundamental emission as
a precursor to dust formation.
