@Kasumi_Scarlet idk

I18n.format is not necessary...
and if a variable is null, then it instanceof MyClass is always false.

if(potioneffect.getAmplifier() == 0) s1 = s1 + " I"; else if (potioneffect.getAmplifier() == 1) s1 = s1 + " II"; else if (potioneffect.getAmplifier() == 2) s1 = s1 + " III"; else if (potioneffect.getAmplifier() == 3) s1 = s1 + " IV"; else if (potioneffect.getAmplifier() == 4) s1 = s1 + " V"; else if (potioneffect.getAmplifier() == 5) s1 = s1 + " VI"; else if (potioneffect.getAmplifier() == 6) s1 = s1 + " VII"; else if (potioneffect.getAmplifier() == 7) s1 = s1 + " VIII"; else if (potioneffect.getAmplifier() == 8) s1 = s1 + " IX"; else if (potioneffect.getAmplifier() == 9) s1 = s1 + " X"; else if (potioneffect.getAmplifier() >= 10) s1 = s1 + " X+"; else s1 = s1 + "¿";

in this part I think it's better to use switch

Ah, yes,
Thanks for telling me

Some explanation of code

Following code in JavaScript equals number > 0 ? Math.floor(number) : Math.ceil(number)

number |= 0 ~~number

But according to my test, ~~x is much faster than Math.floor(x) for random positive double value. So you can use bit operation instead of Math.floor(x) and parseInt(x)

if (block != null) blocks.set(blockPos, block); can be simplified to if (block) blocks.set(blockPos, block); or just block && blocks.set(blockPos, block);
Because null in JavaScript is considered as false value, and && operator will not do things after it if things before it is a false value.