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

April 13, 2022

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());

>Solution :

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());