javascript.reduce() - reduce group

reduceGroupNumbers.js

const reduceOccurrences = manyNumbers.reduce((acc, cur) => {
  acc[cur] ? acc[cur]++ : acc[cur] = 1
  return acc
}, {})

Noticed a typo:

What's the point of the if? It works fine without it:

manyNumbers = [1, 1, 2, 3, 3, 3, 4, 4, 5, 5, 5, 5, 5, 6, 7, 7, 7, 7]
manyNumbers.reduce((acc, cur) => {
  acc[cur] ? acc[cur]++ : acc[cur] = 1
  return acc
}, {})
// => {1: 2, 2: 1, 3: 3, 4: 2, 5: 5, 6: 1, 7: 4}

I think the if must have been left over by mistake when someone converted the if/else from the beginner version in https://github.com/p-j-anderson/blog--javascript-reduce/blob/master/grouping.js into a ternary operator. The ternary operator is the replacement for the if/else. So making that expression be the condition of another if statement is a mistake.

You should always return a sane accumulator value from each iteration of a reduce loop. Otherwise you can run into an error like TypeError: Cannot read property '2' of undefined in the next iteration.

The code as-is is equivalent to:

const reduceOccurrences = manyNumbers.reduce((acc, cur) => {
  if (acc[cur] ? acc[cur]++ : acc[cur] = 1) {
    return acc
  } else {
    // Don't return anything. The accumulator will be undefined in the next iteration.
  }
}, {})

It only gets away with it because the if condition is always a nonzero number and therefore always truthy... :)

Good catch! You're probably correct on the hypothesis of how it got there. Fixed!