Destructuring assignment allow us to assign properties of arrays or objects to variables.
function foo(x = {y: 10}, {y = 20} = {}) {
console.log(x.y, y);
}
foo(); // 10 20
foo({y: 30}, {y: 40}); // 30 40
foo({}, {}); // undefined 20The inconsistency is on the last line with foo({}, {})'s result of undefined 20.
Our first argument {} is able to override the default value of {y: 10}. x.y logs undefined.
However, our second argument {} is not able to override our variable y and not having it declared.
The inconsistency comes because the syntax is quite similar but the behaviour varies depending on the way we do destructuring.