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

// Test handling of false return from a handler.set() hook.

load(libdir + "asserts.js");

var obj = {x: 1};
var p = new Proxy(obj, {
    set(target, key, value, receiver) { return false; }
});

// Failing to set a property is a no-op in non-strict code.
assertEq(p.x = 2, 2);
assertEq(obj.x, 1);

// It's a TypeError in strict mode code.
assertThrowsInstanceOf(() => { "use strict"; p.x = 2; }, TypeError);
assertEq(obj.x, 1);

// Even if the value doesn't change.
assertThrowsInstanceOf(() => { "use strict"; p.x = 1; }, TypeError);
assertEq(obj.x, 1);

// Even if the target property doesn't already exist.
assertThrowsInstanceOf(() => { "use strict"; p.z = 1; }, TypeError);
assertEq("z" in obj, false);

// [].sort() mutates its operand only by doing strict [[Set]] calls.
var arr = ["not", "already", "in", "order"];
var p2 = new Proxy(arr, {
    set(target, key, value, receiver) { return false; }
});
assertThrowsInstanceOf(() => p2.sort(), TypeError);
assertDeepEq(arr, ["not", "already", "in", "order"]);