Use bs.new to emulate e.g. new Date():

type t
external createDate : unit -> t = "Date" [@@bs.new]

let date = createDate ()

type t;
[@bs.new] external createDate : unit => t = "Date";

let date = createDate();

Output:

var date = new Date();

You can chain bs.new and bs.module if the JS module you're importing is itself a class:

type t
external book : unit -> t = "Book" [@@bs.new] [@@bs.module]
let bookInstance = book ()

type t;
[@bs.new] [@bs.module] external book : unit => t = "Book";
let bookInstance = book();

Output:

var Book = require("Book");
var bookInstance = new Book();

JS classes are really just JS objects wired up funnily. In general, prefer using the previous object section's features to bind to a JS object.

OCaml having classes really helps with modeling JS classes. Just add a [@bs] to a class type to turn them into a Js.t class:

class type _rect = object
  method height : int [@@bs.set]
  method width : int [@@bs.set]
  method draw : unit -> unit
end [@bs]

type rect = _rect Js.t

class type _rect =
  [@bs]
  {
    [@bs.set] pub height: int;
    [@bs.set] pub width: int;
    pub draw: unit => unit
  };

type rect = Js.t(_rect);

For Js.t classes, methods with arrow types are treated as real methods (automatically annotated with [@bs.meth]) while methods with non-arrow types are treated as properties. Adding bs.set to those methods will make them mutable, which enables you to set them using #= later. Dropping the bs.set attribute makes the method/property immutable. Basically like the object section's features.