To base64 encode a JavaScript object, we convert it to a JSON string and base64 encode that.
For instance, we write
const obj = { a: "a", b: "b" };
const encoded = btoa(JSON.stringify(obj));
const actual = JSON.parse(atob(encoded));
to call JSON.strignify to convert obj to a JSON string.
Then we call btoa to convert the JSON string into a base64 string.
And we convert the base64 string back to an object with atob and JSON.parse.
