Occasionally, you have to break out of your normal cljs/FP flow and access whatever object this refers to. In most cases, you’ll need to do so because:

  1. You’re interacting with a JS lib
  2. You’re using jQuery (e.g. ($ this))

cljs provides the this-as macro to access this. When initially writing a line-reader, I used this-as to access/set data on the transformer:

;;interacting with a JS lib
(defn- transform [chunk encoding done]
  (this-as this ;;it's idiomatic to name it `this`
    (let [data (if (.-_lastLineData this)
                 (str (.-_lastLineData this) chunk)
                 (str chunk))
          lines (clojure.string/split data (js/RegExp. eol "g"))]
      (set! (.-_lastLineData this) (last lines))
      (doseq [line (butlast lines)]
        (.push this line))

(defn- flush [done]
  (this-as this
    (if (.-_lastLineData this)
      (.push this (.-_lastLineData this)))
    (set! (.-_lastLineData this) nil)

This is a departure from normal cljs in that we’re mutating the object referred to by this.

With React libraries like Om, Reagent, and Quiescent, I’ve used jQuery less and less. It’s rare that I need to access this, but cljs makes it easy to do so when necessary.

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