scientist.js

:microscope: A JavaScript interpretation of the Ruby library Scientist, a library for carefully refactoring critical paths.

How do I science?

Let's pretend you're changing the way you handle permissions in a large web app. Tests can help guide your refactoring, but you really want to capture the current and refactored beahviours under load.

import {Experiment, Result} from 'scientist.js';

import Permission from './permission';
import Model from './model';

export default class MyWidget {
    constructor() {
        this.model = new Model();
    }

    allows(user) {
        var experiment = new Experiment("widget-permissions");
        experiment.use(() => this.model.checkUser(user).valid); // old way
        experiment.try(() => user.can(Permission.Read, this.model)); // new way

        return experiment.run();
    }
}

Use use(..) to wrap the existing original behaviour, and use try(..) to wrap the new behaviour. experiment.run(); will always return the result of the use block, but it does a bunch of stuff behind the scenes:

• It decides whether or not to run the try block
• Randomises order to execute try and use blocks
• Measures the duration of both behaviours
• Swallows (but records) any exceptions raised in the try block and
• Set a condition to filter calls to try

Upcoming features (these already exist in the Ruby library):

• Compares the result of try to the result of use,
• Publishes all this information.

The use block is called the control. The try block is called the candidate.