Javascript Tips: Add patterns for mapping and lookups

The JS Tip section could use a bit about mapping and lookup patterns.

For mapping, I'd like to show example usage of array map and reduce, with short simple examples.

For lookups (dictionaries), I'd like to show a lookup pattern to map static values, functions or templates.

This is basically nonsense pseudocode but this is the sort of thing I want to show:

const map = {
   'static-key:' 'some-other-value',
   'dynamic-key': (v) => v.toUpperCase()
}

const mapped = patients.map((patient) => {
  const p = {};
  for (key of patient) {
   const mappedKey = map[key].(key)   ?? key;
   p[mappedKey] = map[patient[key]] ?? patient[key]
  }
 return p;
});

Start with the JS Tip section at documentation/jobs/javascript and review its existing examples and style. Add short array map and reduce examples plus lookup examples for static values, functions, and templates; it is done when all requested mapping and lookup patterns are clearly documented there.