We perform 2D resistive magnetohydrodynamic simulations of coronal jets driven by flux emergence along the lower boundary. The reconnection layers are susceptible to the formation of blobs that are ejected in the jet. Our simulation with low plasma beta (Case I) shows that magnetic islands form easily and propagate upward in the jet. These islands are multithermal and thus are predicted to show up in hot channels (335 angstrom and 211 angstrom) and the cool channel (304 angstrom) in observations by the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory. The islands have maximum temperatures of 8 MK, lifetimes of 120 s, diameters of 6 Mm, and velocities of 200 km s(-1). These parameters are similar to the properties of blobs observed in extreme-ultraviolet (EUV) jets by AIA. The Kelvin-Helmholtz instability develops in our simulation with moderately high plasma beta (Case II) and leads to the formation of bright vortex-like blobs above the multiple high magnetosonic Mach number regions that appear along the jet. These vortex-like blobs can also be identified in the AIA channels. However, they eventually move downward and disappear after the high magnetosonic Mach number regions disappear. In the lower plasma beta case, the lifetime for the jet is shorter, the jet and magnetic islands are formed with higher velocities and temperatures, the current-sheet fragments are more chaotic, and more magnetic islands are generated. Our results show that the plasmoid instability and Kelvin-Helmholtz instability along the jet are both possible causes of the formation of blobs observed at EUV wavelengths.