Unsoundness in coercion from object to boolean

[uhyo]

TypeScript Version: 3.4.3, typescript@3.5.0-dev.20190518

Search Terms: unsound coercion object boolean

Code

const obj: { toFixed: unknown } = 0;
// val has type "123"
const val = obj && "123";
console.log(val.slice()); // <- runtime error!

Expected behavior:
Compile error.

Actual behavior:
This code compiles and causes a runtime error when run.

TypeScript assumes that any value with object type (except {}) is truthy, but it's not true as illustrated above. If one of 0, "", false or 0n is assignable to an object type, it should not be considered truthy.

Playground Link: https://www.typescriptlang.org/play/#src=const%20obj%3A%20%7B%20toFixed%3A%20unknown%20%7D%20%3D%200%3B%0A%2F%2F%20val%20has%20type%20%22123%22%0Aconst%20val%20%3D%20obj%20%26%26%20%22123%22%3B%0Aconsole.log(val.slice())%3B%0A%0A%0A

Related Issues: #18196 (fixed by #27157)

[lll000111]

Unsoundness in coercion from object to boolean

Isn't the unsoundness the very first line

const obj: { toFixed: unknown } = 0;

Coercing a primitive (number) to an object?

[shqld]

At a single glance it seems TypeScript should distinguish number from Number instance?

E.g. when we annotate a certain variable as Number specifically, TS should regard it as an object with some props & methods it should have but otherwise as if it'd be a real primitive value same as other languages.

[MartinJohns]

Coercing a primitive (number) to an object?

It is not coercing a primitive to an object. The type { toFixed: unknonw } does not constrain to objects. It just says whatever is passed has a toFixed property.

If you want to enforce having an object, then you can make use of the object type:

const obj1: { toFixed: unknown } & object = 0; // Does not compile
const obj2: { toFixed: unknown } & object = { toFixed: 0 }; // Does compile

[lll000111]

@MartinJohns

Is this bad a joke?

It just says whatever is passed has a toFixed property.

Which means it is an object. Primitives don't have properties. Your comment is factually sooo wrong... do we really need to discuss the very basics of JS? (Rhetorical question)

> 1.toFixed()
VM43:1 Uncaught SyntaxError: Invalid or unexpected token

> Number(1).toFixed()
"1"

@uhyo Why do you upvote factually obviously wrong stufff? Just curious. Please learn the very basics of the language, it seems you have serious gaps if you think primitives in JS have properties.

https://javascriptweblog.wordpress.com/2010/09/27/the-secret-life-of-javascript-primitives/

[MartinJohns]

I was not precise enough. It doesn't has to be a property, but anything with the name 'toFixed', and numbers have a function named like that.

const n = 0;
console.log(n.toFixed);

[lll000111]

@MartinJohns

but anything with the name 'toFixed',

OPs example has a primitive number!! Not an object! Primitive number 0 is not an object without coercion, in this language!

I really don't see the point of discussing language basics. You can learn all about primitives and objects in JS in free introductory Javascript courses.

[MartinJohns]

My example has a primitive number as well. I just omitted the type information. My example does the same as his: store 0 in a variable.

[lll000111]

Which is coerced to an object.

You do understand JS automatically does type coercion?

> typeof 0.toFixed
VM134:1 Uncaught SyntaxError: Invalid or unexpected token

> typeof Number(0).toFixed
"function"

A primitive number is not an object. Yes, you can coerce it to be one, news at 11. I thought you use static typing to not use such language features.

https://javascriptweblog.wordpress.com/2010/09/27/the-secret-life-of-javascript-primitives/

See under

If primitives have no properties, why does "abc".length return a value?

Because JavaScript will readily coerce between primitives and objects.
In this case the string value is coerced to a string object in order to access the property length. The string object is only used for a fraction of second after which it is sacrificed to the Gods of garbage collection

Your example explained right there, only using a string primitive instead of a number primitive.

Enough already, I'm out.

[RyanCavanaugh]

@lll000111 take it down a notch; we don't need to be condescending to have a discussion.

Anyway, as to the OP: Types written as { p: T; } are not constrained to be object-like. There is plenty of code that operates well on things that have length and are indexable (including strings) or things that have a toString (including numbers). This does create a soundness gap for the falsy primitive values, but it's not with any palatable solutions.