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"
