Arrow Functions

In short: (params) => expression is a slimmer way to write a function — but it's not just shorthand. An arrow function has no this and no arguments of its own; both are borrowed from the surrounding scope where the arrow was written. That makes arrows the right choice inside a callback that needs the outer this, and the wrong choice for an object method or a constructor.

The same function, written two ways

For a plain calculation with no this involved, the two are interchangeable:

Both give 10. The arrow drops function and return entirely when the body is a single expression — pure convenience, nothing else changes yet.

Fixing this inside a callback

The real reason arrows exist: a regular function passed to setTimeout gets its own this, which is not the object you meant.

Because the arrow has no this of its own, this inside it means exactly what it meant one line above, in incArrow — counter. A regular function passed to setTimeout instead would get a this decided by how setTimeout calls it, not by where it was written — almost never what you want.

Borrowing arguments too

Arrows don't just skip their own this — they skip their own arguments object as well, and see the enclosing function's instead:

inner is an arrow, so arguments inside it isn't its own — it reaches out to regularOuter's arguments, which has length 3. An arrow function genuinely has no arguments binding of its own to shadow it.

Quick practice