Why Use ECMA 5 as Basis?
The third reason for not using ECMA6+ is that it seems to be built with the idea that programs will be written with a framework (React, Angular, Vue, Ember, yada, yada, yada). This is not a book about any of those frameworks or frameworks in general. The intent is to teach CS students CS principals and philosophies. Students can learn a framework, anyone one of which will be relegated to legacy status is 5 years, after they graduate.