If like me, you are just learning MakeCode for Minecraft, you will need a Sandbox World that is as simple as can be. This video shows how to make and use a Sandbox world without respecifying it every time.
A simple world is particularly important when using coordinates, for example, in a flat world the ground is always in the same up-down (Y) position. Further, when I am about to test my code for the umpteenth time, I don’t want it to start raining or becoming night. You’ll find it much easier to root out those inevitable programming bugs without such distractions. I also turn off the music and lower the other sounds.
It may seem contradictory that I leave the randomly spawned animals — I like the company even if Slimes can be a nuisance.