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describe("[[Set]] trap normal behavior", () => {
test("forwarding when not defined in handler", () => {
expect((new Proxy({}, { set: undefined }).foo = 1)).toBe(1);
expect((new Proxy({}, { set: null }).foo = 1)).toBe(1);
expect((new Proxy({}, {}).foo = 1)).toBe(1);
});
test("correct arguments supplied to trap", () => {
let o = {};
let p = new Proxy(o, {
set(target, prop, value, receiver) {
expect(target).toBe(o);
expect(prop).toBe("foo");
expect(value).toBe(10);
expect(receiver).toBe(p);
return true;
},
});
p.foo = 10;
});
test("conditional return value", () => {
let p = new Proxy(
{},
{
set(target, prop, value) {
if (target[prop] === value) {
target[prop] *= 2;
} else {
target[prop] = value;
}
},
}
);
p.foo = 10;
expect(p.foo).toBe(10);
p.foo = 10;
expect(p.foo).toBe(20);
p.foo = 10;
expect(p.foo).toBe(10);
p[Symbol.hasInstance] = "foo";
expect(p[Symbol.hasInstance]).toBe("foo");
});
test("custom receiver value", () => {
const o = {};
const r = {};
let p = new Proxy(o, {
set(target, property, value, receiver) {
receiver[property] = value;
return true;
},
});
expect(Reflect.set(p, "foo", 42, r)).toBe(true);
expect(o.foo).toBeUndefined();
expect(r.foo).toBe(42);
});
});
describe("[[Set]] invariants", () => {
test("cannot return true for a non-configurable, non-writable property", () => {
let o = {};
Object.defineProperty(o, "foo", { value: 10 });
let p = new Proxy(o, {
set() {
return true;
},
});
expect(() => {
p.foo = 12;
}).toThrowWithMessage(
TypeError,
"Proxy handler's set trap violates invariant: cannot return true for a property on the target which is a non-configurable, non-writable own data property"
);
});
test("cannot return true for a non-configurable accessor property with no setter", () => {
let o = {};
Object.defineProperty(o, "foo", { get() {} });
let p = new Proxy(o, {
set() {
return true;
},
});
expect(() => {
p.foo = 12;
}).toThrowWithMessage(
TypeError,
"Proxy handler's set trap violates invariant: cannot return true for a property on the target which is a non-configurable own accessor property with an undefined set attribute"
);
});
});
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