Why is this an issue?

Functions declared with the set keyword will automatically return the values they were passed. Thus any value explicitly returned from a setter will be ignored, and explicitly returning a value is an error.

Noncompliant code example

var person = {
  // ...
  set name(name) {
    this.name = name;
    return 42;  // Noncompliant
  }
}

Compliant solution

var person = {
  // ...
  set name(name) {
    this.name = name;
  }
}