JSZip uses objects that may not exist on every platform, in which case it uses a shim. Accessing or replacing these objects can sometimes be useful. JSZip.external contains the following properties :
Promise : the Promise implementation used.The global object is preferred when available.
Example
// use bluebird instead
JSZip.external.Promise = Bluebird;
// use the native Promise object:
JSZip.external.Promise = Promise;