It is strange that the author has not included the use of Promises and the async keyword. Should be essential knowledge today.

> Value vs. Reference — Understand how objects, arrays, and functions are copied and passed into functions. Know that the reference is what's being copied. Understand that primitives are copied and passed by copying the value.

> Scope — Understand the difference between global scope, function scope, and block scope. Understand which variables are available where. Know how the JavaScript engine performs variable lookup.

I'd say the above two are essential to know in 99% of programming languages out there.