Arrays in JavaScript have several methods for filtering, mapping, or folding that require a callback. Not having a return statement in such a callback function is most likely a mistake, because processing of the array uses the return value of the callback. If there is no return, callback will implicitly return ``++undefined++``, which will likely fail.


This rule applies for the following methods of an array:

* ``++Array.from++``
* ``++Array.prototype.every++``
* ``++Array.prototype.filter++``
* ``++Array.prototype.find++``
* ``++Array.prototype.findIndex++``
* ``++Array.prototype.map++``
* ``++Array.prototype.reduce++``
* ``++Array.prototype.reduceRight++``
* ``++Array.prototype.some++``
* ``++Array.prototype.sort++``


== Noncompliant Code Example

----
let arr = ["a", "b", "c"];
let merged = arr.reduce(function(a, b) {
  a.concat(b);
}); // Noncompliant: No return statement, will result in TypeError 
----


== Compliant Solution

----
let arr = ["a", "b", "c"];
let merged = arr.reduce(function(a, b) {
  return a.concat(b);
}); // merged === "abc"
----



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