variable.md

Variables

You can try the code below in Lumo.

To define a variable:

(def a 1)
; #'cljs.user/a

Or to create a local binding:

(let [x 1
      y 2]
  (+ x y))
; 3

Normally, you don't modify the value of a variable. To define a variable to a changing state, use the type Atom:

(def b (atom 1))
; #'cljs.user/b

A reference to the Atom is returned. To read the value of the reference, use @b, which means (deref b). We prefer to name the variable as *b to indicates it's changing state. And to change the value:

(def *b (atom 1))
; #'cljs.user/b
(reset! *b 2)
; 2

There's another use case in building apps. To cooperate with hot code swapping, we use defonce to persist the state during code swapping:

(defonce *c (atom 1))
; #'cljs.user/*c