TypeError: Reduce of empty array with no initial value (V8-based & Firefox & Safari)

In JavaScript, there are several reduce functions:

• Array.prototype.reduce(), Array.prototype.reduceRight() and
• TypedArray.prototype.reduce(), TypedArray.prototype.reduceRight()).

These functions optionally take an initialValue (which will be used as the first argument to the first call of the callback). However, if no initial value is provided, it will use the first element of the Array or TypedArray as the initial value. This error is raised when an empty array is provided because no initial value can be returned in that case.

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.

const 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:

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

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.

const 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 to handle the empty case, either before calling reduce, or in the callback after adding an unexpected dummy initial value.

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

let nameList1 = "";
if (names.length >= 1) {
  nameList1 = Array.prototype.reduce.call(
    names,
    (acc, name) => `${acc}, ${name}`,
  );
}
// nameList1 === "" when names is empty.

const nameList2 = Array.prototype.reduce.call(
  names,
  (acc, name) => {
    if (acc === "")
      // initial value
      return name;
    return `${acc}, ${name}`;
  },
  "",
);
// nameList2 === "" when names is empty.

• Array.prototype.reduce()
• Array.prototype.reduceRight()
• TypedArray.prototype.reduce()
• TypedArray.prototype.reduceRight()
• Array
• TypedArray
• Array.prototype.filter()
• TypedArray.prototype.filter()