Docs / BuckleScript / NullUndefinedOption

Null, Undefined & Option

BuckleScript itself doesn't have the notion of null or undefined. This is a great thing, as it wipes out an entire category of bugs. No more undefined is not a function, and cannot access foo of undefined!

However, the concept of a potentially nonexistent value is still useful, and safely exists in our language.

We represent the existence and nonexistence of a value by wrapping it with the option type. Here's its definition from the standard library:

type option('a) = None | Some('a)

It means "a value of type option is either None (nothing) or that actual value wrapped in a Some". You might wonder why we can't drop that Some wrapper and just do the following:

type option('a) = None | 'a

So that the value can be either None or theActualValue. While convenient, there are many reasons why this is undesirable later on. We'll spare the explanation here.


Here's a normal value:

let licenseNumber = 5

To represent the concept of "maybe null", you'd turn this into an option type by wrapping it. For the sake of a more illustrative example, we'll put a condition around it:

let licenseNumber = if (personHasACar) { Some(5); } else { None; };

Later on, when another piece of code receives such value, it'd be forced to handle both cases:

switch (licenseNumber) { | None => print_endline("The person doesn't have a car") | Some(number) => print_endline("The person's license number is " ++ string_of_int(number)) };

By turning your ordinary number into an option type, and by forcing you to handle the None case, the language effectively removed the possibility for you to mishandle, or forget to handle, a conceptual null value!


The Option helper module is here.

Interoperate with JavaScript undefined and null

The option type is common enough that we special-case it when compiling to JavaScript:


simply compiles down to 5, and


compiles to undefined! If you've got e.g. a string in JavaScript that you know might be undefined, type it as option(string) and you're done! Likewise, you can send a Some(5) or None to the JS side and expect it to be interpreted correctly =)

Caveat 1

The option-to-undefined translation isn't perfect, because on our side, option values can be composed:


This still compiles to 5, but this gets troublesome:


This is compiled into the following JS:


What's this Js_primitive thing? Why can't this compile to undefined? Long story short, when dealing polymorphic option type (aka option('a), for any 'a), many operations become tricky if we don't mark the value with some special annotation. If this doesn't make sense, don't worry; just remember the following rule:

  • Never, EVER, pass a nested option value (e.g. Some(Some(Some(5)))) into the JS side.

  • Never, EVER, annotate a value coming from JS as option('a). Always give the concrete, non-polymorphic type.

Caveat 2

Unfortunately, lots of times, your JavaScript value might be both null or undefined. In that case, you unfortunately can't type such value as e.g. option(int), since our option type only checks for undefined and not null when dealing with a None.

Solution: More Sophisticated undefined & null Interop

To solve this, we provide access to more elaborate null and undefined helpers through the Js.Nullable module. This somewhat works like an option type, but is different from it.


To create a JS null, use the value Js.Nullable.null. To create a JS undefined, use Js.Nullable.undefined (you can naturally use None too, but that's not the point here; the Js.Nullable.* helpers wouldn't work with it).

If you're receiving, for example, a JS string that can be null and undefined, type it as:

[@bs.module "MyConstant"] external myId: Js.Nullable.t(string) = "myId"

To create such a nullable string from our side (presumably to pass it to the JS side, for interop purpose), do:

[@bs.module "MyIdValidator"] external validate: Js.Nullable.t(string) => bool = "validate"; let personId: Js.Nullable.t(string) = Js.Nullable.return("abc123"); let result = validate(personId);

The return part "wraps" a string into a nullable string, to make the type system understand and track the fact that, as you pass this value around, it's not just a string, but a string that can be null or undefined.

The .t in Js.Nullable.t is an OCaml convention. Js.Nullable.t denotes the primary data type that module Js.Nullable operates on. Most of the time, this t type is also usually an abstract type i.e. you do not know (and should not have to know) the internal representation of t. However, in the case of Js.Nullable.t('a), you can guess that it's something a long the line of type Js.Nullable.t('a) = Js.Nullable.null | Js.Nullable.undefined | Some('a).

Convert to/from option

Js.Nullable.fromOption converts from a option to Js.Nullable.t. Js.Nullable.toOption does the opposite.