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?
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));
};
readAs* is defined to be asynchronous, which means the onloadend function will be defined before the event is called.
I know this is kind of old but for someone looking for a better and newer solution please use the Crypto API and a SHA-256 or higher variant for the algorithm since MD5 has exploitable flaws.
var a = new FileReader();
a.readAsArrayBuffer(blob);
a.onloadend = function () {
let hashPromise = crypto.subtle.digest("SHA-256", a.result);// it outputs a promise
};
