What means this comma separated callback return value expression

Question

I have the following code:

const data = [
  [{
      id: 1,
      done: false
    },
    {
      id: 2,
      done: true
    }
  ],
  [{
      id: 3,
      done: false
    },
    {
      id: 4,
      done: false
    }
  ],
  [{
      id: 5,
      done: true
    },
    {
      id: 6,
      done: false
    }
  ],
  [{
      id: 7,
      done: true
    },
    {
      id: 8,
      done: false
    }
  ]
]

//pV=previous value, cV=current value, cI=current index
const result = data.reduce((pV, cV, cI) => (cV.some(obj => obj.done) && pV.push(cI), pV), []);

console.log(result);

I understand the way how reduce works, but I don't get it how the return value of callback is composed here:

(cV.some(obj => obj.done) && pV.push(cI), pV)

I mean yes it checks if cV has some object where done is true and pushes cI to pV(the accumulator), but why is pV comma separated at the end? How does this expression return a pV array?

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comma_Operator — epascarello, Oct 27 '20 at 00:25
It's a very confusing way of returning the `pV` using the comma operator. Better to avoid it. — CertainPerformance, Oct 27 '20 at 00:25
@epascarello In minified code, sure, but not in *source* code, at least in my experience - in which it looks quite confusing (hence this question and many others) and is often forbidden by a project's linting rules — CertainPerformance, Oct 27 '20 at 00:27
`function(pV, cV, cI) { if (cV.some( function(obj) { return obj.done } )) { pV.push(cI); } return pV; }` — epascarello, Oct 27 '20 at 00:28

What means this comma separated callback return value expression

0 Answers0