brainCloud's Cloud Code is based on Apache Rhino, which is an open-source implementation of JavaScript. This engine is highly sandboxed within the brainCloud API servers for security purposes.
brainCloud currently ships with Apache Rhino version 1.7.7.1.
Rhino implements JavaScript 1.7, and is basically ES4 with partial ES5 and ES6 support.
Summary from a Stack Overflow discussion here:
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Hope that helps!
Paul.