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Errors: Reduce of empty array with no initial value

Message

TypeError: reduce of empty array with no initial value

Error type

TypeError

What went wrong?

In JavaScript, there are several reduce functions:

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.

Examples

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.