ClojureScript transitions from ES3 (1999) to ES2016. Major JS VMs now offer great performance across the spec, enabling Proxy, Reflect, and modern iteration protocols.

Brain teaser angle: Code that relied on ES3 polyfill behavior may break or behave differently under ES2016 semantics.

New namespace uses ES2016 Proxy to bridge ClojureScript maps and vectors to JavaScript objects and array-likes, reducing clj->js marshalling:

(require '[cljs.proxy :refer [builder]])
(def proxy (builder))
(def proxied-map (proxy {:foo 1 :bar 2}))

Brain teaser angle: Proxied maps look like JS objects to JS code but are still persistent ClojureScript maps underneath. Mutations on the JS side don't propagate.

Supports method value syntax with ES2016 Reflect eliminating manual :param-tags requirements:

(refer-global :only '[String])
(map String/.toUpperCase ["foo" "bar" "baz"])
;; => ("FOO" "BAR" "BAZ")

• :refer-global declares global definitions available without js/ prefix
• :require-global enables using JS libraries via script tags without build configuration

Two compiler flags for smaller artifacts:

• :lite-mode – simpler 2011-era data structures, ~2/3 size reduction
• :elide-to-string – removes toString implementations for tree-shaking

Example: a functional composition program compiles to 6K Brotli compressed.

Brain teaser angle: Lite mode data structures may have different performance characteristics and missing protocol implementations compared to full ClojureScript. (= (type []) cljs.core/PersistentVector) could return false in lite mode.