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.
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
var merged = arr.reduce(function(a, b) {
a.concat(b);
}); // Noncompliant: No return statement
var merged = arr.reduce(function(a, b) {
return a.concat(b);
});