src/third_party/mozjs-45/js/src/jit-test/tests/proxy/testDirectProxySet7.js

// Assigning to a proxy with no set handler calls the defineProperty handler
// when no such property already exists.

var hits = 0;
var t = {};
var p = new Proxy(t, {
    defineProperty(t, id, desc) { hits++; return true; }
});
p.x = 1;
assertEq(hits, 1);
assertEq("x" in t, false);

// Same thing, but the receiver is a plain object inheriting from p:
var receiver = Object.create(p);
hits = 0;
receiver.x = 2;
assertEq(hits, 0);
assertEq("x" in t, false);
assertEq(receiver.hasOwnProperty("x"), true);
assertEq(receiver.x, 2);