How to read Blob synchronously in Javascript?

I’m writing a chrome extension to intercept WebSocket traffic, by hooking the function WebSocket.send, like:

var _send = WSObject.send
WSObject.send = function (data) {
    arguments[0] = my_hook_function(data)
    _send.apply(this, arguments)
}

function my_hook_function(data) {
    // how to read the data here synchronously?
    return data
}

The my_hook_function should be synchronous. The type of data is Blob, I can’t find a synchronous API for reading the Blob. Google says there is FileReaderSync but I tried in console and this class does not exist.

How to solve it?