API / Js

Js

The Js module mostly contains ReScript bindings to standard JavaScript APIs like console.log, or the JavaScript String, Date, and Promise classes.

It is meant as a zero-abstraction interop layer and directly exposes JavaScript functions as they are. If you can find your API in this module, prefer this over an equivalent Belt helper. For example, prefer Js.Array2 over Belt.Array

Argument Order

For historical reasons, some APIs in the Js namespace (e.g. Js.String) are using the data-last argument order whereas others (e.g. Js.Date) are using data-first.

For more information about these argument orders and the trade-offs between them, see this blog post.

Eventually, all modules in the Js namespace are going to be migrated to data-first though.

In the meantime, there are several options for dealing with the data-last APIs:

Examples

RESCRIPT
/* Js.String (data-last API used with pipe last operator) */
Js.log("2019-11-10" |> Js.String.split("-"))
Js.log("ReScript" |> Js.String.startsWith("Re"))

/* Js.String (data-last API used with pipe first operator) */
Js.log("2019-11-10"->Js.String.split("-", _))
Js.log("ReScript"->Js.String.startsWith("Re", _))

/* Js.String (data-last API used without any piping) */
Js.log(Js.String.split("-", "2019-11-10"))
Js.log(Js.String.startsWith("Re", "ReScript"))

Js.Xxx2 Modules

Prefer Js.Array2 over Js.Array, Js.String2 over Js.String, etc. The latters are old modules.

t

RESCRIPT

type t<'a> = 'a
  constraint 'a = {..}

JS object type

null

RESCRIPT

type null<'a> = Value('a) | Null

Nullable value of this type can be either null or 'a. This type is equivalent to Js.Null.t.

undefined

RESCRIPT

type undefined<+'a>

A value of this type can be either undefined or 'a. This type is equivalent to Js.Undefined.t.

nullable

RESCRIPT

type nullable<'a> = Value('a) | Null | Undefined

null_undefined

RESCRIPT

type null_undefined<'a> = nullable<'a>

toOption

RESCRIPT

let toOption: nullable<'a> => option<'a>

undefinedToOption

RESCRIPT

let undefinedToOption: undefined<'a> => option<'a>

nullToOption

RESCRIPT

let nullToOption: null<'a> => option<'a>

isNullable

RESCRIPT

let isNullable: nullable<'a> => bool

import

RESCRIPT

let import: 'a => promise<'a>

testAny

RESCRIPT

let testAny: 'a => bool

The same as {!test} except that it is more permissive on the types of input

promise

RESCRIPT

type promise<+'a, +'e>

The promise type, defined here for interoperation across packages.

null

RESCRIPT

let null: null<'a>

The same as empty in Js.Null. Compiles to null.

undefined

RESCRIPT

let undefined: undefined<'a>

The same as empty Js.Undefined. Compiles to undefined.

typeof

RESCRIPT

let typeof: 'a => string

typeof x will be compiled as typeof x in JS. Please consider functions in Js.Types for a type safe way of reflection.

log

RESCRIPT

let log: 'a => unit

Equivalent to console.log any value.

log2

RESCRIPT

let log2: ('a, 'b) => unit

log3

RESCRIPT

let log3: ('a, 'b, 'c) => unit

log4

RESCRIPT

let log4: ('a, 'b, 'c, 'd) => unit

logMany

RESCRIPT

let logMany: array<'a> => unit

A convenience function to console.log more than 4 arguments

eqNull

RESCRIPT

let eqNull: ('a, null<'a>) => bool

eqUndefined

RESCRIPT

let eqUndefined: ('a, undefined<'a>) => bool

eqNullable

RESCRIPT

let eqNullable: ('a, nullable<'a>) => bool

unsafe_lt

RESCRIPT

let unsafe_lt: ('a, 'a) => bool

unsafe_lt(a, b) will be compiled as a < b. It is marked as unsafe, since it is impossible to give a proper semantics for comparision which applies to any type

unsafe_le

RESCRIPT

let unsafe_le: ('a, 'a) => bool

unsafe_le(a, b) will be compiled as a <= b. See also Js.unsafe_lt.

unsafe_gt

RESCRIPT

let unsafe_gt: ('a, 'a) => bool

unsafe_gt(a, b) will be compiled as a > b. See also Js.unsafe_lt.

unsafe_ge

RESCRIPT

let unsafe_ge: ('a, 'a) => bool

unsafe_ge(a, b) will be compiled as a >= b. See also Js.unsafe_lt.