Clojure for JavaScript devs

Posted on 2023-04-29. Updated on 2026-07-22.

My random notes about Clojure from the eyes of a JavaScript frontend developer.

Learning resources

• Clojure Documentation: Community-driven documentation. Contains up-to-date practices.
• Caveman - A Clojure Web Framework: Building a web framework from the ground up.
• Learn to build a Clojure web app - a step-by-step tutorial: by Eric Normand.
• Clojure for the Brave and True: My first Clojure book. Funnily written. Maybe outdated.

Installation and Clojure CLI

• Use JDK with SDKMan.

Collections

• Remove items: (filter pred coll), (remove pred coll), (keep f coll). keep will take only non-nil values of (f item), including false.

Hash map

Construct a hash map:

(hash-map :a 1 :b 2)
(into {} [[:a 1] [:b 2]])

into takes a vector of key-value pairs. Can use it for transformation:

(->> [:a :b :c]
  (map-indexed vector)
  (into {}))
;; {0 :a 1 :b 2 :c}

Get item from a map:

(def m {:a 1 :b 2})

(:a m)
(get m :a)
(get m :z "default-value")

Pick some keys from the map:

(select-keys user [:user/name :user/id :user/avatar])

Update keys/ vals of a map:

(update-keys {"a" 1 "b" 2} keyword)
;; {:a 1 :b 2}

(update-vals {:a 1 :b 2} inc)
;; {:a 2 :b 3}

Deploy to Fly.io

First, build the whole project to uberjar file.

To init a project, run:

$ fly launch

Open fly.toml and change services.internal_port. Assume that your web app is running on port 4001:

[[services]]
  protocol = "tcp"
  internal_port = 4001
  processes = ["app"]

Create Dockerfile having EXPOSE the same port value as services.internal_port.

FROM adoptopenjdk:11-jre-hotspot
RUN mkdir /opt/app

COPY target/uberjar/app.jar /opt/app/app.jar

EXPOSE 4001

CMD ["java", "-jar", "/opt/app/app.jar"]

Then

$ fly deploy