It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import at the very top of the file.
Sets the compost value of the given IItemStack.
The amount should be between 0 and 1, anything above 1 would just be wasted.
Return Type: void
// Composter.setValue(stack as IItemStack, amount as float) as void composter.setValue(<item:minecraft:diamond>, 0.85);