Converting from decimals to roman numerals
I would define a constant overline="\u0305" and then use interpolation further down: 4 : `C${overline}D#{overline}`,, it might even make sense to define overlineD and overlineC constants. An alternative might be to create an overline method and write something like 4: overline('CD') or even a tagged template literal (4: overline`CD`) if you want to impress.
Object.keys( algarismsMap ).reverse(): The order of keys in a hash is not guaranteed, it can vary depending on the implementation and if keys have been deleted, etc. If you need order you should use an array. Also rather than reversing just define it backwards.
Rather than if {...} else [lots of code] I find it better to use an early return if {...; return; } [lots of code]
One of the biggest thing to me is the lack of tests.  I would expect to see something in an interview assignment, it doesn't have to use a framework, I would be fine with something like:

function test(number, roman) {
  if (romanizeNumber(number) !== roman)
    console.error(`Failed! ${number} should be ${roman} but was ${romanizeNumber(number)}`);
}
test(10, 'X');
test(11, 'XI');
...