blobtojson.js

//This is how the data looks like at the begginning
//rejection.data: Blob(54) {size: 54, type: "application/json"}

var arrayBuffer;
var fileReader = new FileReader();
fileReader.onload = function() {
    arrayBuffer = this.result;
};

fileReader.readAsArrayBuffer(rejection.data);

var dec= new TextDecoder("utf-8");

var jsonStr= dec.decode(arrayBuffer);

var jsonObj= JSON.parse(jsonStr);

//This is the result: "{"message":"","title":null,"status":500,"code":"1001"}"