src/third_party/mozjs-45/js/src/jit-test/tests/proxy/testDirectProxySetArray3.js

// Assigning to the length property of a proxy to an array
// calls the proxy's defineProperty handler.

var a = [0, 1, 2, 3];
var p = new Proxy(a, {
    defineProperty(t, id, desc) {
        hits++;

        // ES6 draft rev 28 (2014 Oct 14) 9.1.9 step 5.e.i.
        // Since the property already exists, the system only changes
        // the value. desc is otherwise empty.
        assertEq(Object.getOwnPropertyNames(desc).join(","), "value");
        assertEq(desc.value, 2);
        return true;
    }
});
var hits = 0;
p.length = 2;
assertEq(hits, 1);
assertEq(a.length, 4);
assertEq(a[2], 2);