That's called the spread operator.
It unpacks values from an object or array, into another object or array. For example, using arrays:
a1 = [1, 2, 3]
a2 = [4, 5, 6]
a12 = [...a1, ...a2] // [1, 2, 3, 4, 5, 6]
The same semantics apply to objects:
o1 = { foo: 'bar' }
o2 = { bar: 'baz' }
o12 = { ...o1, ...o2 } // { foo: 'bar', bar: 'baz' }
You can use it to shallow-copy objects and arrays:
a = [1, 2, 3]
aCopy = [...a] // [1, 2, 3], on a new array
o = { foo: 'bar' }
oCopy = { ...o } // { foo: 'bar' }, on a new object
Check out the Mozilla docs, an excellent source for all things javascript.
