[jahewson]

Because writing:

    var self = this;
    foo(function(x) {
      return x + self.y;
    });

rather than:

    foo(x => x + this.y)

is so much less error-prone?

[eueueu]

Think about what would happen if you typo a single character in each example.

If you miswrite "function" as "fnuction", you'll easily, quickly find out.

If you miswrite => as >= it might take ages to track it down.

[wereHamster]

That's why you use static type checkers.

If the function `foo` is defined as taking a function as its first argument (or even more precisely, a function which takes a number and returns a number), and you try to give it a boolean, then the static type checker will yell at you, and you'll just as easily find out as that fnuction typo.