gecko-dev/dom/tests/mochitest/fetch/utils.js

// Utilities
// =========

// Helper that uses FileReader or FileReaderSync based on context and returns
// a Promise that resolves with the text or rejects with error.
function readAsText(blob) {
  if (typeof FileReader !== "undefined") {
    return new Promise(function(resolve, reject) {
      var fs = new FileReader();
      fs.onload = function() {
        resolve(fs.result);
      }
      fs.onerror = reject;
      fs.readAsText(blob);
    });
  } else {
    var fs = new FileReaderSync();
    return Promise.resolve(fs.readAsText(blob));
  }
}

function readAsArrayBuffer(blob) {
  if (typeof FileReader !== "undefined") {
    return new Promise(function(resolve, reject) {
      var fs = new FileReader();
      fs.onload = function() {
        resolve(fs.result);
      }
      fs.onerror = reject;
      fs.readAsArrayBuffer(blob);
    });
  } else {
    var fs = new FileReaderSync();
    return Promise.resolve(fs.readAsArrayBuffer(blob));
  }
}