Firebug v2 and ChromeTools know how to parse such JSON and ignore that first part. (IE11 and Firefox newer DevTools can't "handle" it aka show just a plain text string)
Autocorrect of “JS”? If so: it’s not a modern weirdness; this is an old, long-fixed browser bug.
I believe that some browsers used to do that some times ago.
But getting hit with this, they would be actively hurt, and I don't think that interpreter session would be able to recover.
And here's the code in Angular's $http service for stripping out that string:
If your responses are all between 505 and 512 in length then it might matter but most likely you are prematurely optimizing.