Docs / Language Manual / Null, Undefined and Option

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(These docs are equivalent to the old BuckleScript docs before the ReScript rebrand)

Null, Undefined and Option

ReScript 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 someAttribute 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:

Reason (Old Syntax)JS Output
type option('a) = None | Some('a);

It means "a value of type option is either None (representing nothing) or that actual value wrapped in a Some".

Note how the option type is just a regular variant.


Here's a normal value:

Reason (Old Syntax)ML (Older Syntax)JS Output
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:

Reason (Old Syntax)ML (Older Syntax)JS Output
let licenseNumber =
  if (personHasACar) {
  } else {

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

Reason (Old Syntax)ML (Older Syntax)JS Output
switch (licenseNumber) {
| None =>
  Js.log("The person doesn't have a car");
| Some(number) =>
  Js.log("The person's license number is " ++ Js.Int.toString(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! A pure ReScript program doesn't have null errors.

Interoperate with JavaScript undefined and null

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

Reason (Old Syntax)ML (Older Syntax)JS Output
let x = Some(5);

simply compiles down to 5, and

Reason (Old Syntax)ML (Older Syntax)JS Output
let x = None;

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:

Reason (Old Syntax)ML (Older Syntax)JS Output
let x = Some(Some(Some(5)));

This still compiles to 5, but this gets troublesome:

Reason (Old Syntax)ML (Older Syntax)JS Output
let x = Some(None);

What's this Caml_option.some thing? Why can't this compile to undefined? Long story short, when dealing with a 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:

Reason (Old Syntax)ML (Older Syntax)JS Output
[@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:

Reason (Old Syntax)ML (Older Syntax)JS Output
[@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.

Convert to/from option

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