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"