JS Array Reduce 的好奇反应

Question

Been using JS reduce on arrays and I stumbled on this weird undefined response when I used it on an object.

Here is the input:

var desks = [
  { type: "sitting" },
  { type: "standing" },
  { type: "sitting" },
  { type: "sitting" },
  { type: "standing" },
];

and here is the function that am trying to run this input on:

const deskCount = (desks) => {
  desks.reduce(
    (count, thisdesk) => {
      if (thisdesk.type == "sitting") count.sitting = count.sitting + 1;
      if (thisdesk.type == "standing") count.standing = count.standing + 1;
      console.log(count); //this line is logging correctly but function returns undefined
      return count;
    },
    { sitting: 0, standing: 0 }
  );
};

Running it using Node 12.18.0 and wrapped with babel.

Answer 1

The issue is that you have used a multi-line arrow function without returning a value. The inner reduce runs fine, however, the wrapping code does not return anything when it is inside of {} .

Either use return for the wrapping {} , or omit the {} . I think omitting is best.

const deskCount = (desks) =>
  desks.reduce(
    (count, thisdesk) => {
      if (thisdesk.type == "sitting") count.sitting = count.sitting + 1;
      if (thisdesk.type == "standing") count.standing = count.standing + 1;
      console.log(count); //this line is logging correctly but function returns undefined
      return count;
    },
    { sitting: 0, standing: 0 }
  );