What is the output and why?

It prints undefined.

obj.getA() is a method call, where this refers to the object obj, so this.a would be 1.

However, when we assign let getA = obj.getA; and call getA(), the function is invoked as a plain function, not as a method.
In this case, this defaults to:

In non-strict mode → the global object (window in browsers).
In strict mode → undefined.

Since this.a does not exist in the global object and this is undefined (if strict mode), the result is undefined.