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The initial mass function (IMF) is crucial for our understanding of star and galaxy formation and evolution, while our knowledge of the IMF in the early universe remains limited. Recent advancements in numerical simulations have revealed that stellar masses in primordial environments are significantly larger, reaching several hundred solar masses, compared to observations in present-day environments. This suggests the existence of an IMF transition throughout cosmic evolution. Through three-dimensional hydrodynamics simulations, we have discovered that both metallicity and redshift play crucial roles in determining the IMF. Metallicity influences the cooling capability, while the redshift affects the temperature floors by altering the cosmic microwave background temperature. Our simulations have revealed that the IMF becomes top-heavy in environments with Z/Zsun < 0.01 or z > 10. These results may provide an explanation for recent observations by the James Webb Space Telescope (JWST), which have revealed an unexpected abundance of high-redshift luminous galaxies, showing an increase in UV luminosity at fixed stellar mass.