Calculate the hash of Blob using JavaScript

Question

I want to compare two Blobs to see if there are changes between them.

One way of doing this is by calculating the hash of the blobs and then comparing them, e.g.:

hash(firstBlob) === hash(secondBlob)

How can I calculate the hash of a Blob and check against another hash to see if they have changed?

Answer 1

You can use the FileReader API to get the contents of the blob for comparison. If you have to use CryptoJS for this, you can use readAsBinaryString:

var a = new FileReader();
a.readAsBinaryString(blob);
a.onloadend = function () {
  console.log(CryptoJS.MD5(CryptoJS.enc.Latin1.parse(a.result)));
};

---

Note that readAsBinaryString is deprecated, so if you can use another library, such as SparkMD5, you could use an array buffer instead:

var a = new FileReader();
a.readAsArrayBuffer(blob);
a.onloadend = function () {
  console.log(SparkMD5.ArrayBuffer.hash(a.result));
};