How is this assignment working? (js/ svelte) [duplicate]

I’m following the svelte tutorial and I don’t see how this assignment on line 10 of the tick page in chapter 7 linked here is working.

 const { selectionStart, selectionEnd, value } = this;

this doesn’t have selectionStart and selectionEnd defined at this point in the code, and when it does assign them it uses those values, which seems like a loop to me. I just don’t see where they’re coming from or how this doesn’t produce an error.