Doesn not work correctly for numbers over 18 digits.
Example

e = 555555555555555555555555555555555555555555555555
e.toLocaleString() 

// "555,555,555,555,555,600,000..."

This is nice except that every single one of 4 ifs will be tested even though at most 1 can be truthy.

Why? You are reducing to a single number according to the data ops. The only time there is no operation on that number is for operation o , which just pushes the number to the main function's output array. Seems pretty straightforward.
I've done it a bit differently in that my reduce function builds the main output array but I like this top solution, focusing on the number itself.

that should be ilegal LMAO

Using reduce as a quasi loop without even returning any reduced value is kind of ugly to my taste.

Insightful! 🏆

good