parsing json in a worker

parseJson.js

// promisified version of parsing json in a worker
// usage:  `let data = await fetch(url).then(v => v.text()).then(v => parseJson(v)).then(data => console.log(data));`
function parseJson(txt:string) {
  return new Promise((resolve, reject) => { 
    var blob = new Blob([
      'this.onmessage = function(message) {\n' +
        'postMessage(JSON.parse(message.data));\n' +
      '};'
      ], { type: "text/javascript" });
    var w = new Worker(URL.createObjectURL(blob));
    w.onmessage = (message) => resolve(message.data);
    w.postMessage(txt);
  })
}