observe-path

Object.defineProperty based path observation, in ES6 style.

observe-path is a property change observation library designed to allow explicit binding of property change callbacks to a given path from a root object.

It is written in ES6, so needs to be transpiled using (babel)[https://babeljs.io] or (traceur)[https://github.com/google/traceur-compiler], or a similar tool if your browser or (Node.js)[https://nodejs.org/] does not yet support the required ES6 features.

observe-path is designed for explicit path binding - it cannot observe property additions, or arbitrary array modifications, the path bound must be known prior to observation. The path can, however, be broken, reformed, or changed in any way and continue to provide change notifications.

Object.defineProperty is used to observe changes at each path segment, for several reasons:

• It is fast in most current JavaScript engines
  • simple microbenchmarks vs direct property access give ~350% execution time for reading observed properties in v8 and IonMonkey, and ~800% execution time for writing observed properties (with a single empty change callback function).
  • with the exception of Chakra in IE11-, rough microbenchmarking shows 3 orders of magnitude difference there.
• It is less surprising than the event loop turn based Object.observe, giving immediate response capability.
• It is more widely available than Object.observe, which, at the time of writing is only available in v8 and not polyfillable, Object.defineProperty is supported pretty much universally.

observe-path is highly cooperative - it respects existing property descriptors, passing through set operations to existing setters, fetching from existing getters, and tidying up behind itself when observation is no longer required.

It is also designed to be efficient and fast when paths are applied many times, caching common state on prototypes, front-loading the work of observation to the declaration point - to keep actual observation lean.

Install

npm install observe-path

Testing

Change your working directory to the observe-path directory, and ensure dev dependencies are installed.

cd node_modules/observe-path
npm install

To run nodejs tests:

npm test

To perform web browser tests:

npm test web

And visit the testem URL provided.

Usage

Import the library, create a Path instance, and observe that path from a specified root object:

import Path from 'observe-path/lib/path';

var path = new Path(['a', 'b', 'c', someSymbol]);

path.observe(someObject, (value, old) => {
	// React to value change.
});

The change callback will be invoked on any property change, with the new and previous values supplied. This callback is guaranteed to only be called once per value change, even if intermediary path segments change, but the final value does not (if you wish to hear about intermediary property changes, observe them instead!).