When the ScriptEngineMajorVersion function is called, it returns a value that identifies the major revision level of the implementation, not the revision level of the ECMAScript or JScript language specification that is currently supported by the implementation. This return value cannot be used as a reliable indicator of the availability or lack of availability of specific language features.

The JScript 5.x implementations within Windows Internet Explorer 7 and Windows Internet Explorer 8 always return a value of 5.