index.js

var bundleFn = arguments[3];
var sources = arguments[4];
var cache = arguments[5];

var stringify = JSON.stringify;

module.exports = function (fn) {
    var keys = [];
    var wkey;
    var cacheKeys = Object.keys(cache);

    for (var i = 0, l = cacheKeys.length; i < l; i++) {
        var key = cacheKeys[i];
        var exp = cache[key].exports;
        // Using babel as a transpiler to use esmodule, the export will always
        // be an object with the default export as a property of it. To ensure
        // the existing api and babel esmodule exports are both supported we
        // check for both
        if (!exp) {
            continue;
        }
        if (exp === fn || exp.default === fn) {
            wkey = key;
            break;
        }
    }

    if (!wkey) {
        wkey = Math.floor(Math.pow(16, 8) * Math.random()).toString(16);
        var wcache = {};
        for (var i = 0, l = cacheKeys.length; i < l; i++) {
            var key = cacheKeys[i];
            wcache[key] = key;
        }
        sources[wkey] = [
            Function(['require','module','exports'], '(' + fn + ')(self)'),
            wcache
        ];
    }
    var skey = Math.floor(Math.pow(16, 8) * Math.random()).toString(16);

    var scache = {}; scache[wkey] = wkey;
    sources[skey] = [
        Function(['require'], (
            // try to call default if defined to also support babel esmodule
            // exports
            'var f = require(' + stringify(wkey) + ');' +
            '(f.default ? f.default : f)(self);'
        )),
        scache
    ];

    var src = '(' + bundleFn + ')({'
        + Object.keys(sources).map(function (key) {
            return stringify(key) + ':['
                + sources[key][0]
                + ',' + stringify(sources[key][1]) + ']'
            ;
        }).join(',')
        + '},{},[' + stringify(skey) + '])'
    ;

    var URL = window.URL || window.webkitURL || window.mozURL || window.msURL;

    return new Worker(URL.createObjectURL(
        new Blob([src], { type: 'text/javascript' })
    ));
};