Item and Ingredient

When making recipes you can specify items in many ways, the most common is just to use 'namspace:id', like 'minecraft:diamond', however you can also use Item#of and Ingredient#of for advanced additions, such as NBT or count.

Note that Item and Ingredient are not the same! They may work similarly but there are differences. Item can only ever represent a single item type whereas Ingredient can represent multiple item types (and multiple instances of the same item type with different properties such as NBT data). For most cases Ingredient should be preferred over Item.

Item/ItemWrapper

Its Java class name is ItemWrapper but it is bound to Item in JS.

Name	Return Type	Info
of(ItemStackJS in)	ItemStackJS	Returns an ItemStackJS based on what was input.  Note that this relies mostly on Rhinos type wrapping to function, see paragraph below about ItemStackJS#of for more info
of(ItemStackJS in, int count)	ItemStackJS	See above. count will override any other count set from the first parameter.
of(ItemStackJS in, CompoundTag tag)	ItemStackJS	See above. nbt is merged, with the input nbt taking priority over existing nbt.
of(ItemStackJS in, int count, CompoundTag nbt)	ItemStackJS	Combines the functionality of the above two.
withNBT(ItemStackJS in, CompoundTag nbt)	ItemStackJS	Same as the corresponding #of.
withChance(ItemStackJS in, double chance)	ItemStackJS	Same as #of, chance will override currently set chance.
getList()	ListJS	Returns a list of ItemStackJS, one per registered item.
getTypeList()	ListJS	Returns a list of String, one per registered item.
getEmpty()	ItemStackJS	Returns ItemSTackJS.EMPTY
clearListCache()	void	Clears the caches used for #getList and #getTypeList
fireworks(Map<String, Object> properties)	FireworkJS	Returns a FireworkJS based on the input map of propeties. See FireworkJS#of on the FireworkJS page for more information <TODO: Make and link FireworkJS page>
getItem(ResourceLocation id)	Item	Returns the instance of the Item class associated with the item id passed in.
@Nullable findGroup(String id)	CreativeModTab	Returns the Creative tab associated with the id passed in, returns null if none found.
exists(ResourceLocation id)	boolean	Returns if the item id passed in exists or not.
isItem(@Nullable Object o)	boolean	Just does an instanceof ItemStackJS check on the object passed in.

Item#of relies on Rhinos type wrapping to function, which calls ItemStackJS#of. This tries its best to turn the input into an ItemStackJS. If no match is found ItemStackJS.EMPTY is returned. Valid inputs:

• null/ItemStack.EMPTY/Items.EMPTY/ItemStackJS.EMPTY - will return ItemStackJS.EMPTY
• ItemStackJS will return the same object passed in.
• FluidStackJS - will return a new DummyFluidItemStackJS 
• IngredientJS - will return the first item in the Ingredient
• ItemStack - Will return a new ItemStackJS wrapping the ItemStack passed in
• ResourceLocation  - Will lookup this ResourceLocation in the item registry and return it if found. If not found will return ItemStackJS.EMPTY, and throw an error if RecipeJS.itemErrors is true
• ItemLike - Will return a new ItemStackJS of the input
• JsonObject - Will return an item based on properties in the Json. item will be used as the item id, or tag if item does not exist. count, chance and nbt all set their respective properties
• RegEx - Will return a new ItemStackJS of the first item id that matches this regex.
• String (CharSequence) - Will parse it and return a new ItemStackJS based on the input item id. Prefix with nx  to change the count (where n is any number between 1 and 64). Put # before the item id to parse it as a tag instead. Put @ before the item id to parse it as a modid instead. Prefix with % to parse it as a creative menu tab group. Surround in / to parse as a RegEx. NOTE: will only be the first item in any of the groups mentioned above!
• Map/JS Object - Uses the same rules as a JsonObject.

Ingredient/IngredientWrapper

Its Java class name is IngredientWrapper but it is bound to Ingredient in JS.

Name	Return Type	Info
getNone()	IngredientJS	Returns ItemStack.EMPTY
getAll()	IngredientJS	Returns an IngredientJS of every single item in game. All of them.
of(Object object)	IngredientJS	Works exactly the same as Item#of except it recognises Ingredient and forge json ingredient syntax.
of(Object object, int count)	IngredientJS	Same as above. The count passed in will override any from the first parameter.
custom(Predicate<ItemStackJS> predicate)	IngredientJS	Takes the arrow function or anonymous function passed in and makes an IngredientJS with that as #test. Return true from the function if the ItemStackJS passed should match as an ingredient.
custom(IngredientJS in, Predicate<ItemStackJS> predicate)	IngredientJS	Same as above except it must match the IngredientJS passed in as the first parameter before the custom function is called.
customNBT(IngredientJS in, Predicate<CompoundTag> predicate)	IngredientJS	Same as above except the Predicate is passed the items nbt instead of the full ItemStackJS. Useful for advanced NBT matching.
matchAny(Object objects)	IngredientJS	Adds the passed in object to an ingredient. If it is a list then it adds all items in the list. All objects are passed through #of before adding.
registerCustomIngredientAction(String id, CustomIngredientActionCallback callback)	void	Registers a custom ingredient action. See the recipe page for more information.
isIngredient(@Nullable Object o)	boolean	Just does an instanceof IngredientJS check on the object passed in.

Remember that Item and Ingredient are not equivalent!