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describe("[[Get]] trap normal behavior", () => {
test("forwarding when not defined in handler", () => {
expect(new Proxy({}, { get: undefined }).foo).toBeUndefined();
expect(new Proxy({}, { get: null }).foo).toBeUndefined();
expect(new Proxy({}, {}).foo).toBeUndefined();
});
test("correct arguments supplied to trap", () => {
let o = {};
let p = new Proxy(o, {
get(target, property, receiver) {
expect(target).toBe(o);
expect(property).toBe("foo");
expect(receiver).toBe(p);
},
});
p.foo;
});
test("conditional return", () => {
let o = { foo: 1 };
let p = new Proxy(o, {
get(target, property, receiver) {
if (property === "bar") {
return 2;
} else if (property === "baz") {
return receiver.qux;
} else if (property === "qux") {
return 3;
}
return target[property];
},
});
expect(p.foo).toBe(1);
expect(p.bar).toBe(2);
expect(p.baz).toBe(3);
expect(p.qux).toBe(3);
expect(p.test).toBeUndefined();
expect(p[Symbol.hasInstance]).toBeUndefined();
});
test("custom receiver value", () => {
let p = new Proxy(
{},
{
get(target, property, receiver) {
return receiver;
},
}
);
expect(Reflect.get(p, "foo", 42)).toBe(42);
});
});
describe("[[Get]] invariants", () => {
test("returned value must match the target property value if the property is non-configurable and non-writable", () => {
let o = {};
Object.defineProperty(o, "foo", { value: 5, configurable: false, writable: true });
Object.defineProperty(o, "bar", { value: 10, configurable: false, writable: false });
let p = new Proxy(o, {
get() {
return 8;
},
});
expect(p.foo).toBe(8);
expect(() => {
p.bar;
}).toThrowWithMessage(
TypeError,
"Proxy handler's get trap violates invariant: the returned value must match the value on the target if the property exists on the target as a non-writable, non-configurable own data property"
);
});
test("returned value must be undefined if the property is a non-configurable accessor with no getter", () => {
let o = {};
Object.defineProperty(o, "foo", { configurable: false, set(_) {} });
let p = new Proxy(o, {
get() {
return 8;
},
});
expect(() => {
p.foo;
}).toThrowWithMessage(
TypeError,
"Proxy handler's get trap violates invariant: the returned value must be undefined if the property exists on the target as a non-configurable accessor property with an undefined get attribute"
);
});
});
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