Javascript question on .reduce(), cannot figure out a syntax

Question

I am learning .reduce(), and come up with using .reduce to reduce the pairs array to a single object with the key-value totals.

There is a line I don't understand.

Here is the code:

function GroupTotals(strArr) {
  // Create an empty object to store the key-value totals
  var totals = {};
  
  // Split each string in strArr by ":" and parse the value as an integer
  // Then, create an array of [key, value] pairs for each string
  var pairs = strArr.map(s => s.split(/:/)).map(s => [s[0], parseInt(s[1])]);
  
  // Reduce the pairs array to a single object with the key-value totals
  totals = pairs.reduce((acc, s) => (acc[s[0]] = (acc[s[0]] || 0) + s[1], acc), {});
  
  // Create an array of keys from the totals object, sort them alphabetically,
  // filter out any keys with a total value of 0, and map the remaining keys
  // to "key:total" strings
  var output = Object.keys(totals).sort().filter(k => totals[k] !== 0).map(k => `${k}:${totals[k]}`);
  
  // Join the output array with commas to form the final output string
  return output.join(',');
}

I don't understand why in the line, that acc at the end of the line ,acc),{}) mean "return acc".

totals = pairs.reduce((acc, s) => (acc[s[0]] = (acc[s[0]] || 0) + s[1], acc), {});

Correct me if I am wrong, I think it is the same as

function sumPairs(acc, s) {
  acc[s[0]] = (acc[s[0]] || 0) + s[1];
  return acc;
}

const totals = pairs.reduce(sumPairs, {});

Please explain why ,acc mean "return acc". I tried to look up in mdn syntax, but couldn't figure it out.

Hope it make sense and thank you in advance. This is the first time I ask a question in stack flow.