Is callback function essentially a function inside another function?

why is this wrong:

markImportant({ Id: this.row.Id })
          .then(this.showToast("success"))
          .catch(this.showToast("error"));

And this is right?

markImportant({ Id: this.row.Id })
      .then(() => {
          this.showToast("success");
      })
      .catch(() => {
          this.showToast("error");
      });

I’m passing a function as a paremeter in both cases. The first case is only one function, whereas the second function is a function inside another function as a parameter.

>Solution :

in .then(this.showToast("success")) you are invoking the showToast function which i believe it returns undefined so you can’t invoke undefined

in then(() => { this.showToast("success")}) you are passing a function which can be invoked so thats why that works