push() won't work as expected in reduce()

Question

Why doesn\'t a.push(b) work in my Array.reduce()? a=a.push(b) where b is a string, turns a to an integer.?!

Answer 1

Building on the excellent answer by @Alexander Moiseyev.

You can avoid setting and mutating both variables a and winner by doing the following:

return objKeys.reduce((acc, val) =>
    frequency[val] === highestVal 
      ? [...acc, val] 
      : acc
,[])

note: for clarity I have explicitly declared the accumulator and value in this reduce method.