this ended up being too slow (given the overhead of spinning up mocha with babel). I could probably spin up mocha in memory (with babel) and re-run that way and be fast again... not really worth it til I have a lot more files so just using --watch for now

testing only one file.js

var gaze = require('gaze');
var Rx = require('rxjs');
var spawn = require('child_process').spawn;

// Watch all .js files/dirs in process.cwd()
gaze([
        './src/**/*.js',
        './src/*.js'
    ],
    function(err, watcher) {
        const getAllWatchedFiles = this.watched.bind(this);

        this.on('all', function(event, filepath) {
            const testFilePath = getTestFilePathFor(filepath, getAllWatchedFiles());
            if (testFilePath){
                console.log('deteched change in', filepath, 'testing with', testFilePath);
                var child = spawn('node_modules/.bin/mocha', ['--compilers', 'js:babel-register', testFilePath], { stdio : 'inherit'});
            }
        });
        console.log('watching for file changes');
    }
);


function getTestFilePathFor(filePath, allWatchedFilePaths){
    const flatFilePaths = flatten(allWatchedFilePaths);

    // if it is a test file return it
    if (filePath.endsWith('.tests.js')){
        return filePath;
    }

    // if it has a test file, return the test file
    const testFilePath = filePath.replace(/\.js$/, '.tests.js');
    return flatFilePaths.indexOf(testFilePath) !== -1 ? testFilePath : null;
}


function flatten(allWatchedFilePaths){
    return Object.keys(allWatchedFilePaths).reduce(function (accumulator, key) {
        return accumulator.concat(allWatchedFilePaths[key]);
    }, []);
}