了解箭头功能参数

I just practiced a standard basic whiteboard problem: Create an array filled with integers from 0 through n. My solution worked but there were others posted that had very unusual syntax. I am really trying to understand it but the MDN docs aren't helping me very much. I can put together how {length: n} works but (_, i) => i seems strange. _ is the unnamed function and it takes in i and returns i? but why is that there? I would love any help.

我的解决方案:

function arr(n){
  var newArr = [];
  for(var i = 0; i < n; i++){
    newArr.push(i);
  }
  return newArr;
}

新的语法解决方案:

const arr = n => Array.from({length: n}, (_, i) => i);