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
type t<'a> = 'a
constraint 'a = {..}JS object type
null
type null<'a> = Value('a) | NullNullable value of this type can be either null or 'a. This type is equivalent to Js.Null.t.
undefined
type undefined<+'a>A value of this type can be either undefined or 'a. This type is equivalent to Js.Undefined.t.
nullable
type nullable<'a> = Value('a) | Null | Undefinednull_undefined
type null_undefined<'a> = nullable<'a>toOption
let toOption: nullable<'a> => option<'a>undefinedToOption
let undefinedToOption: undefined<'a> => option<'a>nullToOption
let nullToOption: null<'a> => option<'a>isNullable
let isNullable: nullable<'a> => boolimport
let import: 'a => promise<'a>testAny
let testAny: 'a => boolThe same as {!test} except that it is more permissive on the types of input
promise
type promise<+'a, +'e>The promise type, defined here for interoperation across packages.
null
let null: null<'a>The same as empty in Js.Null. Compiles to null.
undefined
let undefined: undefined<'a>The same as empty Js.Undefined. Compiles to undefined.
typeof
let typeof: 'a => stringtypeof x will be compiled as typeof x in JS. Please consider functions in
Js.Types for a type safe way of reflection.
log
let log: 'a => unitEquivalent to console.log any value.
log2
let log2: ('a, 'b) => unitlog3
let log3: ('a, 'b, 'c) => unitlog4
let log4: ('a, 'b, 'c, 'd) => unitlogMany
let logMany: array<'a> => unitA convenience function to console.log more than 4 arguments
eqNull
let eqNull: ('a, null<'a>) => booleqUndefined
let eqUndefined: ('a, undefined<'a>) => booleqNullable
let eqNullable: ('a, nullable<'a>) => boolunsafe_lt
let unsafe_lt: ('a, 'a) => boolunsafe_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
let unsafe_le: ('a, 'a) => boolunsafe_le(a, b) will be compiled as a <= b.
See also Js.unsafe_lt.
unsafe_gt
let unsafe_gt: ('a, 'a) => boolunsafe_gt(a, b) will be compiled as a > b.
See also Js.unsafe_lt.
unsafe_ge
let unsafe_ge: ('a, 'a) => boolunsafe_ge(a, b) will be compiled as a >= b.
See also Js.unsafe_lt.
