1.0.3 • Public • Published


Wrap an object to track property assignments and method calls made on the wrapper directly. All such assignments and calls are forwarded to the underlying object, and an array of all such assignments and calls can be retrieved. This array can also be used to playback the same actions on a different object.


If run in a browser

var canvas1 = document.createElement("canvas");
var context1 = canvas1.getContext("2d");
var Tracker = require("object-track");
var trackedContext = Tracker.track(context1);
trackedContext.strokeStyle = "#FF0000";
trackedContext.moveTo(5, 5);
trackedContext.lineTo(10, 10);
// Draws a red diagonal line in context1.
var actions = Tracker.getActions(trackedContext); /* actions is [
{key: "strokeStyle", set: "#FF0000"},
{key: "beginPath", arguments: []},
{key: "moveTo", arguments: [5, 5]},
{key: "lineTo", arguments: [10, 10]},
{key: "stroke", arguments: []},
var canvas2 = document.createElement("canvas");
var context2 = canvas2.getContext("2d");
Tracker.play(context2, actions);
// Draws a red diagonal line in context2.


var Tracker = require("object-track");


Returns a wrapper of the object that records all property assignments and method calls before forwarding them. The tracked properties and methods include those found by a for-in loop run on the object at the time this method was called. Extensions to the object are not tracked; see the limitations section for details.


Pass a return of Tracker.track. Returns an array of objects representing all property assignments and method calls made on the tracker.

  • Property assignments are recorded with keys key and set, where key is a string naming the property assigned to and set is the value that was assigned.
  • Method calls are recorded with keys key and arguments, where key is a string naming the method called and arguments is an array with all arguments passed (and not an array-like object).

Since all arrays are truey, the easiest way to distinguish between the two kinds of actions is if (action.arguments).

Tracker.play(object, actions)

The second parameter should be a return of Tracker.getActions or any object in that format. The appropriate property assignments and method calls are made on the provided object in order. Returns the object.


With npm installed, run

npm install object-track


With npm installed, run

npm test

To lint with ESLint, run

npm run check

Edge Cases and Limitations

tl;dr: Complex object interactions and dynamic objects are likely to break the tracker. Tracking is best done on objects with no external relationships and a fixed set of properties and methods, like a canvas context.

If a method call is made on the tracker with a context other than the tracker itself, the call is assumed to operate on a different object and is not recorded in the actions list. The call is forwarded as usual, however.

The tracker is intentionally shallow, and only calls made directly on it will be tracked. If the wrapped object has methods that call its other methods, those internal calls are not made on the tracker and will not be recorded. Example:

var Tracker = require("object-track");
var obj = {
  method1: {},
  method2: {
var tracked = Tracker.track(obj);
// [{key: "method2", arguments: []}]
// Notice the call to method1 is not recorded.

The environment is assumed not to support proxies or object observation, so the implementation uses nothing more complicated than Object.defineProperty, and a for-in loop is used to detect properties and methods to track. The consequences of this are

  • nonenumerable properties and methods are not in the tracker, and interactions with them as though the original object was being used may fail.
  • delete object[key] operations are not tracked and will result in that key no longer being tracked or forwarded.
  • if the object's properties contain objects themselves, mutations or method calls on them will not be tracked.
  • additions of new properties or methods to the tracked object are not tracked.
  • additions of new properties or methods to the tracker will not be forwarded.

In theory, the nonenumerable properties issue could be fixed by using Object.getOwnPropertyNames and a walk up the prototype chain. This may become an option in a future version.

Finally, objects with a key __TRACKING_DATA__ won't work correctly. (This key is used internally by the tracker.)




npm i object-track

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  • mellowmelon