javascript - 谁能解释一下这段代码:

Question

我已经得到了这段代码，但无法获取 r.concat 部分，因为 concat 通常用于整个数组而不是其中的单个元素.

function doubleOddNumbers(numbers) {
  return numbers.reduce((r, n) => n % 2 ? r.concat(n * 2) : r, [])
}

Answer 1

下面是注释的代码:

function doubleOddNumbers(numbers) {
  return numbers.reduce( // reduce iterates over numbers and passes an accumulator from iteration to iteration
    (r, n) => // the reducer function called for each element, r is the accumulator, n is the element
      n % 2   // if the element is odd
        ? r.concat(n * 2) // then append its double to the accumulator
        : r   // otherwise return the accumulator unchanged
  , [])       // start with an empty array for the accumulator
}

这里是关于 reduce 的 MDN 文档和concat .