Clojure doesn't hide the underlying host from the developer. It lets you bypass Clojure and write code close to the metal. This chapter on interop is focused on JavaScript as a host, but most of the concepts will apply to other hosts too. The code examples in this chapter are in the first-project.interop namespace.

js namespace#

JavaScript objects like Array, Date, Math, and Object are accessible via the js/ namespace. You can evaluate these forms and check what they look like:

js/Math ;; => #js {}

js/Array ;; => #object[Array]

(type js/Array) ;; => #object[Function]

In a browser environment, you can access the window object using js/window.

Instantiating objects#

To create instances of JS objects, we can use the new function or the . dot suffix:

(new js/Date) ;; => #inst "2020-08-14T06:39:35.529-00:00"

(js/Date.) ;; => #inst "2020-08-14T06:39:49.126-00:00"

All arguments passed are passed down to the constructor:

(new js/Array 1 2 3) ;; => #js [1 2 3]

(js/Array. 1 2 3) ;; => #js [1 2 3]

Notice the #js prefix in the result of the inline evaluation. This signifies that the data structure is not a CLJS vector but a JS Array.

The forms (new js/Foo p q r) and (js/Foo. p q r) are equal to new Foo(p, q, r);. You can use Klipse to check the transpilation in real-time.

Calling object methods#

A method bar on object Foo can be called using (.bar Foo p q r). This is equal toFoo.bar(p, q, r) in the JS domain:

;; same as Date.now()

(.now js/Date) ;; => 1597388702083

;; same as (new Array(1, 2, 3)).map(i => i + 1)

(.map (new js/Array 1 2 3) inc)

The now method on the js/Date object is a static function, ie doesn't require an initialized object. The .map method on js/Array is an instance method, ie requires an instance of Array to operate on.

A static method bar defined for a class Foo can be alternatively called using (js/Foo.bar). We can rewrite the now function call as:

(js/Date.now) ;; => 1597389177815