exactOptionalPropertyTypes does not apply to constructor parameters

[sxxov]

Bug Report

🔎 Search Terms

exact, optional, property, types, constructor, parameters, arguments, undefined, ?

🕗 Version & Regression Information

4.4 & above (including the latest nightly, v4.7.0-dev.20220504)

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about optional class members
• I was unable to test this on prior versions because exactOptionalPropertyTypes was added in 4.4

⏯ Playground Link

Playground link with relevant code

💻 Code

class A {
    public prop?: 'bar';

    constructor(
        public ctorProp?: 'foo',
    ) {}
}

// correctly throws an error
new A().prop = undefined;

// incorrectly(?) doesn't throw an error, even though a developer might expect it to
new A().ctorProp = undefined;

🙁 Actual behavior

Only the property which is defined on the class itself reports as an error, when undefined is assigned to them.

// correctly throws an error
new A().prop = undefined;

// incorrectly(?) doesn't throw an error, even though a developer might expect it to
new A().ctorProp = undefined;

The types on the class is incorrect as well.

// 'bar'
new A().prop;

// 'bar' | undefined
new A().ctorProp;

In the JS emitted, one can see that an assignment is made regardless if a parameter is passed into it.

class A {
    ctorProp;
    prop;
    constructor(ctorProp) {
        this.ctorProp = ctorProp;
    }
}

While typescript's error is consistent with the outputted code, this may not be the expected behaviour when exactOptionalPropertyTypes is enabled. Related: #47350.

🙂 Expected behavior

Properties defined in both styles should report an error, when undefined is assigned to them.

// correctly throws an error
new A().prop = undefined;

// correctly throws an error
new A().ctorProp = undefined;

The types on the class should have their behaviours matched as well.

// 'bar'
new A().prop;

// 'bar'
new A().ctorProp;

The JS emitted, while I'm not sure how viable it is, should only assign when a parameter is passed in.

class A {
    ctorProp;
    prop;
    constructor(ctorProp) {
        // if target is >= es2021
        this.ctorProp ??= ctorProp;

        // else
        // if (0 in arguments) this.ctorProp = ctorProp;
    }
}

[MartinJohns]

IMO this is a duplicate of #44548. Or at the very least it would first require #44548.

exactOptionalPropertyTypes does not apply for parameters. You can pass undefined as a value, and then you'd require additional logic to be emitted - something that TypeScript opted out quite a while ago.

[sxxov]

Oops, could not find #44548 due to the setting being changed from --strictOptionalProperties to --exactOptionalPropertyTypes.

Though I don't think it's 100% a duplicate, I understand the viewpoint & don't believe this issue can be fixed without additional logic. It still is a big thorn in DX though imo when using the ctor params pattern.