My code works fine in modern browsers but doesn't work in IE11. I removed all the lambdas of the expression, but I have problems with that. How can I rewrite this piece of code in ES5 to avoid destructuring?

.then(function(response)  {
       var reader = response.body.getReader();
       return new ReadableStream({
           start(controller) {
               return pump();
               function pump() {
                   return reader.read().then(function (data) {
                       if (data.done) {
                           controller.close();
                           return;
                       }
                       controller.enqueue(data.value);
                       return pump();
                   });
               }
           },
       });
   })