  The high angular resolution technique of non-redundant masking (NRM) or
aperture masking interferometry (AMI) has yielded images of faint
protoplanetary companions of nearby stars from the ground. AMI on James Webb
Space Telescope (JWST)'s Near Infrared Imager and Slitless Spectrograph
(NIRISS) has a lower thermal background than ground-based facilites and does
not suffer from atmospheric instability. NIRISS AMI images are likely to have
90 - 95% Strehl ratio between 2.77 and 4.8 micron. In this paper we quantify
factors that limit the raw point source contrast of JWST NRM. We develop an
analytic model of the NRM point spread function which includes different
optical path delays (pistons) between mask holes and fit the model parameters
with image plane data. It enables a straightforward way to exclude bad pixels,
is suited to limited fields of view, and can incorporate effects such as
intra-pixel sensitivity variations. We simulate various sources of noise to
estimate their effect on the standard deviation of closure phase, sigma_CP (a
proxy for binary point source contrast). If sigma_CP < 10^-4 radians --- a
contrast ratio of 10 magnitudes --- young accreting gas giant planets (e.g. in
the nearby Taurus star-forming region) could be imaged with JWST NIRISS. We
show the feasibility of using NIRISS' NRM with the sub-Nyquist sampled F277W,
which would enable some exoplanet chemistry characterization. In the presence
of small piston errors, the dominant sources of closure phase error (depending
on pixel sampling, and filter bandwidth) are flat field errors and unmodeled
variations in intra-pixel sensitivity. The in-flight stability of NIRISS will
determine how well these errors can be calibrated by observing a point source.
Our results help develop efficient observing strategies for space-based NRM.
