TypeError: Reduce of empty array with no initial value

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TypeError: 초기값이 없는 빈 배열에 대한 recude는 에러

Error type


What went wrong?

자바스크립크에서 몇 몇의 reduce 함수들:

이러한 함수들은 선택적으로 초기값(initialValue)을 사용합니다.(콜백(callback)의 첫번째 호출에 대한 첫번째 인수로 사용됩니다.) 그러나, 만약에 초기값을 설정하지 않는다면, Array or TypedArray에 대한 첫번째 엘리먼트를 초기값으로 사용 합니다. 이런 경우에 초기값이 없기 때문에 빈 배열이 제공될 경우 오류가 발생 합니다.


Invalid cases

This problem appears frequently when combined with a filter (Array.prototype.filter(), TypedArray.prototype.filter()) which will remove all elements of the list. Thus leaving none to be used as the initial value.

var ints = [0, -1, -2, -3, -4, -5];
ints.filter(x => x > 0)         // removes all elements
    .reduce((x, y) => x + y)    // no more elements to use for the initial value.

Similarly, the same issue can happen if there is a typo in a selector, or an unexpected number of elements in a list:

var names = document.getElementsByClassName("names");
var name_list = Array.prototype.reduce.call(names, (acc, name) => acc + ", " + name);

Valid cases

These problems can be solved in two different ways.

One way is to actually provide an initialValue  as the neutral element of the operator, such as 0 for the addition, 1 for a multiplication, or an empty string for a concatenation.

var ints = [0, -1, -2, -3, -4, -5];
ints.filter(x => x > 0)         // removes all elements
    .reduce((x, y) => x + y, 0) // the initial value is the neutral element of the addition

Another way would be two to handle the empty case, either before calling reduce, or in the callback after adding an unexpected dummy initial value.

var names = document.getElementsByClassName("names");

var name_list1 = "";
if (names1.length >= 1)
  name_list1 = Array.prototype.reduce.call(names, (acc, name) => acc + ", " + name);
// name_list1 == "" when names is empty.

var name_list2 = Array.prototype.reduce.call(names, (acc, name) => {
  if (acc == "") // initial value
    return name;
  return acc + ", " + name;
}, "");
// name_list2 == "" when names is empty.

See also