Why does {. . . .0} evaluate to {}?

Question

I just found {....0} in friend's code. Evaluating it in console returns {} (empty object).

Why is that? What is the meaning of 4 dots in JavaScript?

Answer 1

Four dots actually have no meaning. ... is the spread operator, and .0 is short for 0.0.

Spreading 0 (or any number) into an object yields an empty object, therefore {}.

Answer 2

Three dots in an object literal are a spread property, e.g.:

  const a = { b: 1, c: 1 };   const d = { ...a, e: 1 }; // { b: 1, c: 1, e: 1 } 

The last dot with a 0 is a number literal .0 is the same as 0.0. Therefore this:

 { ...(0.0) } 

spreads all properties of the number object into the object, however as numbers don't have any (own) properties you get back an empty object.