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ClojureScript Data Browser

Developer tooling for inspecting ClojureScript data structures from running programs. Inspect data in devtools using the Chrome or Firefox extension, or in any other browser using the remote inspector.


This is software in the making, not even alpha level. It will have breaking changes. If you're curious, please see below for usage. If you can wait, this tool will at some point become stable, at which point breaking changes will never occur intentionally.

Inspecting a page

You need the small agent library to expose data for inspection. It is available from Clojars:

cjohansen/gadget-inspector {:mvn/version "0.2020.09.10"}

NB! Gadget requires clojure.datafy, and will only work with ClojureScript 1.9.520 or newer.

Then, either create your app-wide atom with the inspector, or inspect an existing atom:

(require '[gadget.inspector :as inspector])

;; Either
(def store (inspector/create-atom "App state" {}))

;; ...or
(def my-store (atom {}))
(inspector/inspect "App state" my-store)

You can also inspect regular persistent data structures, for example the data going into a UI component, with the inspect function:

(def data {:band "Rush"})

(inspector/inspect "Band data" data)

The label operates as both an identifier for your data, and a header, so you can tell structures apart. Inspecting another piece of data with the same label will replace the corresponding section in the inspector.

Controlling the liveness of the inspector

By default, the inspector will only update its UI 250ms after changes to your data occurred. This should help avoid the inspector detracting from your app's performance. You can override this value if you wish:

(require '[gadget.inspector :as inspector])

;; Render less frequently
(inspector/set-render-debounce-ms! 1000)

;; Render synchronously on every change
(inspector/set-render-debounce-ms! 0)

Temporarily pausing inspection

Even with debouncing, you may find that the work done by the inspector is too much in performance critical moments. While this will never be a problem for regular users in production, it can still be annoying during development. For those cases, you can pause the inspector, and resume it when the intensive work has completed:

(require '[gadget.inspector :as inspector]
         '[cljs.core.async :refer [go <!]))


  (<! (animate-smoothly))

The inspector will render immediately when you call resume!.

Inspection filter

Instead of imperatively instructing Gadget to pause and resume all inspections, you can supply a filter that indicates whether or not data should be passed on for inspection. This feature is most useful when you inspect atoms, because those are typically watched and automatically inspected by Gadget. The filter will receive the data to inspect as its only argument (dereferenced, if it is an atom), and should return true to indicate that inspection is desirable:

(require '[gadget.inspector :as inspector])

  "App store"
  (atom {})
  {:inspectable? (fn [state] (not (:page-transition state)))})

Filters also work for non-atom data, which can be useful if you already apply a filter to your inspected atoms, and would like to reuse the logic for one-off inspections:

(require '[gadget.inspector :as inspector])

(def inspection-opts
  {:inspectable? (fn [state] (not (:page-transition state)))})

(inspector/inspect "App store" (atom {}) inspection-opts)

(defn render-app [component page-data element]
  (inspector/inspect "Page data" page-data inspection-opts)
  (render (component page-data) element))

Using the Chrome extension

Start by building the extension. Requires the clojure binary:

make extension

To load up the Chrome extension, open chrome://extensions/, toggle the "Developer mode" toggle in the upper right corner, and then click "Load unpacked" in the upper left corner. Select the extension directory in this repository.

Load your page, open devtools, and then click on the CLJS Data panel. Rejoice. NB! If you've had devtools open since installing the extension, you might need to toggle it once.

Using the Firefox Developer Edition extension

Build with

make firefox.xpi

Firefox requires that extensions are signed, which this obviously is not. So Firefox proper will not allow installation. However the amazing Firefox Developer Edition will.

You need to allow unsigned extensions. This is done in about:config. Toggle xpinstall.signatures.required to false. Then open settings/extensions and find the "Install from file" button. Then select firefox.xpi, and you should be good to go!

Load your page, open devtools, and then click on the CLJS Data panel. Rejoice.

Using the remote inspector

To inspect remotely you'll need to run a small web server. To do this you need Go (eventually we'll ship compiled binaries):

make remote-server

Now open an inspected page in any browser without the extension, and then open http://localhost:7117. It should display a list of app/browser combinations for you to inspect. Click one, and bask in the data glory presented to you.

Teaching Gadget new tricks

Gadget assigns every piece of data a keyword "type" that it uses to dispatch a number of multi-methods to build its UI. By adding new type inferences, you can teach Gadget to use custom rendering and navigation for certain kinds of data - e.g. render "person maps" differently from other maps, render custom literal values in your app, etc.

You can override Gadget's multi-methods for built-in types such as :string as well if you want to change how they are rendered.

Type inferences

New type inferences are added with gadget.datafy/add-type-inference:

(require '[gadget.datafy :as datafy])

  (fn [data]
    (when (:person/id data)

The type inference function is passed a single argument - a piece of data. It should return either a keyword indicating the type of data, or nil to indicate that it does not recognize the type of the data.

When Gadget is identifying the type of some data, it will call these functions starting with the last one added. As soon as it finds a non-nil result, it will use that and stop calling inference functions.

Getting and navigating data

Gadget provides a function for converting values to browsable data and navigate them, just like clojure.datafy.

gadget.core/datafy is a multimethod that converts a value to data. This multi-method allows Gadget to differentiate values based on the synthetic type describe above. While this would be possible through procotol extension by metadata, that would require upfront processing of data to inspect, and would interfere with the plug and play goal of Gadget. Instead, this light abstraction on top of datafy makes it easy to plug in new "types" that increases insight and understand when peering through your data: browsing the claims of a JWT token, or the year, month, day, and other fields of a timestamp. The default implementation of gadget.core/datafy is to call clojure.datafy/datafy.

To navigate data, Gadget uses clojure.datafy/nav directly. Because datafy can associate data with protocols using metadata, there is no need for a dedicated nav implementation in Gadget - just make sure that the return value from datafy implements the Navigable protocol via metadata, if needed.

Gadget passes both the raw and datafyed value to the renderers, so the renderers can use either/or in the visualization. The datafyed version is always used to find navigable keys/indices.

As an example, let's consider how Gadget recognizes and navigates JWTs:

(require '[clojure.string :as str]
         '[gadget.core :as gadget]
         '[gadget.datafy :as datafy])

(def re-jwt #"^[A-Za-z0-9-_=]{4,}\.[A-Za-z0-9-_=]{4,}\.?[A-Za-z0-9-_.+/=]*$")

  (fn [data]
    (when (and (string? data) (re-find re-jwt data))

(defn- base64json [s]
  (-> s js/atob JSON.parse (js->clj :keywordize-keys true)))

(defmethod datafy/datafy :jwt [token]
  (let [[header data sig] (str/split token #"\.")]
    {:header (base64json header)
     :data (base64json data)
     :signature sig}))

(defmethod gadget/render [:inline :jwt] [_ {:keys [raw]}]
  [:strong "JWT: " [:gadget/string raw]])

Let's see this in practice. Given this data:

(def data
  {:token "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c"})

When Gadget renders this map, the token will be rendered with the :inline view. This view is passed both the token string, and the map returned from the datafy implementation. This particular inline view renders the token string with the "JWT: " prefix. Since the datafy result is a map, the token will be navigable in the browser.

When you click the token, Gadget will render the token in the :full view. For :jwt, this view renders the datafyed value with the map browser. Since this is a map, it can be navigated deeper without any further ado.

Rendering custom data types

The first step in rendering custom data types is to define a custom type inference. Let's say we have some literals from the server that we want to preserve with their literal form, such as java.time literals.

Step 1 is to define the type inference:

(require '[gadget.datafy :as datafy])

  (fn [data]
    (when (and (string? data) (re-find #"^#time" data))

The Java time literals are just strings on the client - if we do nothing else, they will be rendered as strings, e.g. "#time/ld \"2019-08-01\"". To make them render as literals, we'll implement the renderer function for inline views of the :java-time-literal type:

(require '[clojure.string :as str]
         '[gadget.core :as gadget])

(defmethod gadget/render [:inline :java-time-literal] [_ {:keys [raw]}]
  (let [[prefix str] (str/split raw " ")]
    [:gadget/literal {:prefix prefix
                      :value (cljs.reader/read-string str)}]))

There is no point in implementing the :full view, because the literal is a string, and strings are not navigable by default, thus it will never be rendered with the :full view.

We could implement datafy for the literal and produce a JS date object, and then use that to power a :full view:

(require '[clojure.string :as str]
         '[gadget.datafy :as datafy])

(defmethod datafy/datafy :java-time-literal [literal]
  (let [[prefix str] (str/split raw " ")]
    (case prefix
      "#time/ld" (let [date-str (cljs.reader/read-string str)
                       [y m d] (map #(js/parseInt % 10) (str/split date-str
                   (js/Date. y (dec m) d)))))

Gadget's built-in instant tooling would pick this up and allow you to navigate into this value to see year, month, date, etc. Please disregard the abomination that is converting a local date to an instant like this.


When adding custom types for maps, you might want a different sorting than the default. By default, Gadget will try to sort keys alpha-numerically. You can override the default behavior by implementing the gadget.core/Browsable protocol. It defines a single method, (entries [d]), which takes the data (e.g. a map), and returns a vector of key value pairs, in preferred order. You can implement this for individual maps by including an implementation as metadata:

(def key-order [:preferred :order :of :keys])

(defmethod gadget.datafy/datafy :my-type [m]
    {`entries (fn [m]
                (sort-by (gadget.core/key-order key-order)))}))


Gadget allows renderers to return pure unadulterated hiccup, essentially allowing you to dictate the exact rendering of a piece of data. To avoid re-implementing components that the Gadget inspector already uses in an uncanny valley all-you-can-eat, Gadget allows you to use some components via namespaced keywords.

Basic types

Dedicated inline components for basic types. All of these take the value as their only argument, e.g:

;; Just the string component
[:gadget/string "Some string"]

;; Embedded in other markup
[:div {}
  [:gadget/string "Some string"]
  [:gadget/number 12]]

The available types are:

  • :gadget/string
  • :gadget/number
  • :gadget/keyword
  • :gadget/boolean
  • :gadget/date


[`:gadget/literal` {:prefix "#inst" :str "2019-01-01T12:00:00Z"}]


Renders a value like a link.

[`:gadget/link` "{100 keys}"]


Inline code samples can be created with [:gadget/code code-string].

Map browser

Most :full renders use the browser component. It takes a sequence of key-value pairs:

[:gadget/browser kv-coll]

kv-coll should be a collection of maps with keys {:k :v :actions} where:

  • :k is hiccup for the key/index
  • :v is hiccup for the value
  • :actions is an actions map - :go and :copy are currently supported


All functions that are exposed for extension.

(defmethod gadget.core/copy type-kw [v])

Returns the value copied to the copy buffer when clicking the copy button next to this value. Dispatches on the synthetic keyword type. The default implementation returns (pr-str v).

(defmethod gadget.core/render [view type] [view data & [view-opts]])

Render the data of type type with view view. Currently view is one of :inline or :full. data is a map with keys :raw - the raw data, :type - the synthetic type, and :data - the datafied data.

(gadget.core/add-type-inference type [f])

Add type inference. Type inference is done by calling each registered function until a non-nil value is returned. LIFO - the last registered inference will be tried first. The function should return nil for unknown values, otherwise a keyword that names the "type". Any other return value will cause an error.

(gadget.datafy/datafy type [val])

A wrapper around clojure.datafy/datafy that dispatches on Gadget's synthtic types. The default implementation calls clojure.datafy/datafy.

Developing Gadget

To develop gadget inspector, clone the repo and open lib/deps.edn. From there, open CIDER (assuming Emacs). If not in Emacs, you should be able to run a Figwheel REPL with:

clojure -A:dev -m figwheel.main --build dev --repl

The [lib/dev/gadget/dev.clj](dev namespace) provides some test data for you to play with. The REPL runs the inspector in a regular browser tab in http://localhost:9570.


Copyright © 2018-2019 Christian Johansen

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.

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