why reduce accumulator doesn't return the intended sum result?

Question

my goal : I want to understand why reduce isn't working as I intended, not how to get the value I expected with other method

my expected results : I expect to have the acc evaluates to sum of 1 in the array but the result I have was 0. yet when I change the acc++ to console.log(item), all item pass the if condition and the value is 1, as expected.

    function testReduce() {
        let a = [1,1,1,1,1,1,1,1,1].reduce((acc, item) => item == 1 ? acc++ : acc,0)
        return a;
    }
    
    console.log(testReduce());

`acc++` won't have any effect as `number` values are not passed by-reference. Postfix increment happens _after_ the rest of the expression runs. Consider using `++acc` or `acc += 1` instead. — Dai, Apr 13 '22 at 06:12
@Dai Or just return `acc + 1` which is the least likely to confuse. — Robby Cornelissen, Apr 13 '22 at 06:16
@RobbyCornelissen Ah yes - I've been getting into the habit of using `+= 1` instead of `++` everywhere ever since Apple removed the `++` operator from Swift (now we just need all the other footgun languages out there to do the same...) — Dai, Apr 13 '22 at 06:17
thanks! yes I did remember learning abt postfix and prefix ++. so this is where it can hurt. — LucasLucas, Apr 13 '22 at 06:31

score 3 · Accepted Answer · answered Apr 13 '22 at 06:13

Post-increment behavior: acc will be returned before it is incremented.

To avoid confusion, just return acc + 1:

function testReduce() {
  let a = [1, 1, 1, 1, 1, 1, 1, 1, 1]
    .reduce((acc, item) => item == 1 ? acc + 1 : acc, 0);
  return a;
}

console.log(testReduce());

why reduce accumulator doesn't return the intended sum result?

1 Answers1