[Enesuygurs]

The *By assert helpers document obj as "object or getter function", with an optional property and an optional message. That covers changesBy, changesButNotBy, increasesBy, increasesButNotBy, decreasesBy, decreasesButNotBy and doesNotDecreaseBy. Passing a getter together with a message doesn't work, though:

let value = 10;
const getter = () => value;
const addTwo = () => { value += 2; };

assert.increasesBy(addTwo, getter, 2);        // passes
assert.increasesBy(addTwo, getter, 2, 'msg'); // fails:
// AssertionError: msg: expected [Function getter] to have property 2

The 4-argument branch (arguments.length === 4 && typeof obj === 'function') moves the delta and the message into place, but it leaves prop set to the delta. .increase(obj, prop) then looks for a property named 2 on the getter. The 3-argument branch right below it already resets prop to null, as do assert.changes / increases / decreases. This change does the same in the 4-argument branch of all seven helpers.

I added getter + message cases to the existing change and increase, decrease tests in test/assert.js. They fail on main with the error above and pass with this change. npm run test-node passes (512 tests), and eslint reports no errors.