Expected to return a value in arrow; function array-callback-return. Why?

Your specific example is:

data.map(users => {
   console.log(users);
});

Where data is the following array:

[
  {id: 1, username: "Foo"},
  {id: 2, username: "Bar"},
]

---

data.map function (check Array.prototype.map specification) converts one array (data in your case) to a new array. The conversion (mapping) is defined by the argument of data.map, also called the callback function. In your case, the callback function is the arrow function users => {console.log(users);}. The callback function of data.map must return a value. By returning a value for each item of the array is how data.map defines the mapping.

But in your case the callback function does not return anything. Your intention is not to do any kind of mapping, but just to console.log. So in your case you can use data.forEach (Array.prototype.forEach) as you don't use data.map functionality.

NOTE: Also you should have singular (rather than plural) name for the parameter of the callback function: data.map(user => {console.log(user);}); as this parameter is set to the individual element from the old array.

If you don't need to mutate the array and just do the console.log() you can do data.forEach() instead. It shouldn't give you the warning. Map expects you to return a value after you've transformed the array.

fetch('/users')
        .then(res => res.json())
        .then(data => {
            data.forEach(users => {
                console.log(users);
            });
        });

Try Changing map(() => {}) to map(() => ())

{} - Creates a code block that expects an explicit return statement.
With () - implicit return takes place.