  Determination of neutrino mass ordering and precision measurement of neutrino
oscillation parameters are the foremost goals of the JUNO experiment. Here, we
explore the effects of scalar non-standard interactions (sNSI) on the electron
neutrino survival probability measured by JUNO. sNSI appears as a correction to
the neutrino mass term in the Hamiltonian. We have considered the simplest
scenario where there is only one NSI ($\eta_{ee}$) present in the theory. Our
results show that sNSI can have significant effect on neutrino oscillation
probabilities at the medium- and long-baseline reactor experiments. We fit
KamLAND data assuming non-zero sNSI in theory and find that {\it estimates of
$\Delta m^2_{21}$ and $\theta_{12}$ from KamLAND deviate significantly from
their standard best-fit values} if one assumes sNSI in the theory. $\eta_{ee}
\in [-1.0, 1.0]$ is allowed by KamLAND. JUNO cannot constrain sNSI but it can
robustly measure $\Delta m^2_{21}$ and $\theta_{12}$ even when they differ
widely from their current best-fit values. {\it Our works highlight the
necessity of global analysis of constraints on sNSI and standard two-flavour
oscillation parameters before arduous three-flavour questions such as neutrino
mass ordering or CP violation in their presence is attempted.
