function shouldBe(func, expected) {
    let result = func();
    if (result !== expected)
        throw new Error("bad value");
}

function shouldThrow(func, errorType, message) {
    let errorThrown = false;
    let error = null;
    try {
        func();
    } catch (e) {
        errorThrown = true;
        error = e;
    }
    if (!errorThrown)
        throw new Error("not thrown");
    if (!(error instanceof errorType))
        throw new Error("wrong error type thrown: " + error);
    if (error.message !== message)
        throw new Error("wrong message thrown: " + error.message);
}

shouldBe(() => isNaN({}), true);
shouldBe(() => isNaN({[Symbol.toPrimitive]: undefined}), true);
shouldBe(() => isNaN({[Symbol.toPrimitive]: null}), true);
shouldBe(() => isNaN({[Symbol.toPrimitive]() { /* empty */ } }), true);
shouldBe(() => isNaN({[Symbol.toPrimitive]() { return NaN } }), true);
shouldBe(() => isNaN({[Symbol.toPrimitive]() { return 1 } }), false);

shouldThrow(() => { isNaN({[Symbol.toPrimitive]: 1 }) }, TypeError, "Symbol.toPrimitive is not a function, undefined, or null");
shouldThrow(() => { isNaN({[Symbol.toPrimitive]: NaN }) }, TypeError, "Symbol.toPrimitive is not a function, undefined, or null");
shouldThrow(() => { isNaN({[Symbol.toPrimitive]: true }) }, TypeError, "Symbol.toPrimitive is not a function, undefined, or null");
shouldThrow(() => { isNaN({[Symbol.toPrimitive]: "string" }) }, TypeError, "Symbol.toPrimitive is not a function, undefined, or null");
shouldThrow(() => { isNaN({[Symbol.toPrimitive]: Symbol() }) }, TypeError, "Symbol.toPrimitive is not a function, undefined, or null");
shouldThrow(() => { isNaN({[Symbol.toPrimitive]: {} }) }, TypeError, "Symbol.toPrimitive is not a function, undefined, or null");
shouldThrow(() => { isNaN({[Symbol.toPrimitive]: [] }) }, TypeError, "Symbol.toPrimitive is not a function, undefined, or null");
shouldThrow(() => { isNaN({[Symbol.toPrimitive]: /regex/ }) }, TypeError, "Symbol.toPrimitive is not a function, undefined, or null");

shouldThrow(() => { isNaN({[Symbol.toPrimitive]() { return this } }) }, TypeError, "Symbol.toPrimitive returned an object");
shouldThrow(() => { isNaN({[Symbol.toPrimitive]() { return {} } }) }, TypeError, "Symbol.toPrimitive returned an object");
shouldThrow(() => { isNaN({[Symbol.toPrimitive]() { return [] } }) }, TypeError, "Symbol.toPrimitive returned an object");
shouldThrow(() => { isNaN({[Symbol.toPrimitive]() { return /regex/ } }) }, TypeError, "Symbol.toPrimitive returned an object");
shouldThrow(() => { isNaN({[Symbol.toPrimitive]() { return function(){} } }) }, TypeError, "Symbol.toPrimitive returned an object");
shouldThrow(() => { isNaN({[Symbol.toPrimitive]() { return Symbol() } }) }, TypeError, "Cannot convert a symbol to a number");
shouldThrow(() => { isNaN({[Symbol.toPrimitive]() { throw new Error("Inner Error") } }) }, Error, "Inner Error");