Spread of optional properties can result in unexpected runtime errors

[issacgerges]

Bug Report

I believe this may just be a gap in how optional interface properties work under the hood, but it's possible to (using spread or assign) produce values that don't match the type systems expectations. You can see that in this example, but in short: Spread/assign are internally check [[GetOwnProperty]] to overwrite a value, so undefined properties can overwrite ones with concrete values. This results in values that don't represent the calculated Intersection.

I think this is caused by how optional properties are represent internally as (at times) T | undefined. Although this holds true for object lookup foo[prop], it does mean that we can create some incorrect values such as this.

interface OptionalFoo {
    foo?: Record<string, number>
}

const undefinedFoo: OptionalFoo = {
    foo: undefined
};

Resulting in errors like this

const spread = {
    foo: {},
    ...undefinedFoo
};

const assigned = Object.assign({}, {foo: {}}, undefinedFoo);

// TS has typed these as `number` but at runtime will produce "Cannot read property 'a' of undefined"
spread.foo['a'];
assigned.foo['a'];

🔎 Search Terms

interface types, optional properties, intersection, spread, assign

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about optional properties and spread/assign

⏯ Playground Link

Playground link with relevant code

💻 Code

(see in description)

🙁 Actual behavior

The type system does not report any errors although the code is guaranteed to produce an error

🙂 Expected behavior

Type system should prevent an optional property from being assigned to undefined unless it was defined as | undefined or the Intersection should contain | undefined

[fatcerberus]

This is the type of thing --strictOptionalProperties was recently introduced to deal with.

[RyanCavanaugh]

With strictOptionalProperties, there's an error on line 6:

interface OptionalFoo {
    foo?: Record<string, number>
}

const undefinedFoo: OptionalFoo = {
    foo: undefined
    ~~~
};

[issacgerges]

Ah, thanks for pointing in the direction @fatcerberus and @RyanCavanaugh . It looks like exactly the gap I believed I found was fixed! I tested against nightly but didn't think to recheck the options. I'll close this one.

[RyanCavanaugh]

You can definitely be forgiven for not knowing about a feature that got checked in a week ago! 😅