Documentation: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax#spread_in_object_literals

Example of such code:

var foo = { a: 123, b: 456 };
var bar = { ...foo, c: 678 };

It seems to be supported by chrome and firefox from 2017 and edge from 2020 so there are more and more libs that use the feature, making them harder to integrate with QML.
Example of such a lib for the sake of having a "complete" repro: https://github.com/tonaljs/tonal