Why does defining a const with a type allow other values than the const when used it as a type?

[dry fable]

Is this because when defining it without a type it becomes a single-value union?
See example - 'c' object allows 'bar' being assigned an invalid value.
https://www.typescriptlang.org/play?#code/MYewdgzgLgBAYiEBhc0YF4YCIBmisDcMA9MTAO4CWUAFiAK6wCmAHgA4A2lw1M0ATpTABzAFChIsAEIBDfikkAuPlEEiM2AEZzCJMjIgxWnbrwFCxoqAE82TeIln8NAb1EwYeEMpt2QOB2RUKAJ3GG1+H1smfxgnBWhQgF9RUQ4mWBllBBAnVzCvbMQEqAAaMIjleODRFLSM8KLcuXyPQpgAci8O8o9KzoiO2tT02GAmvMw3NsRlDoAvRZ69GAAVaM6sRfmsDphKQzAQTIgISmEwGU10mCgQW42O3HwOgDoACgAmAGZPz4BKCpyObbPakCg0az7Qy0A4wGQcDggchMAAmAH5hqkJBAQOlXkjhO8AFIAZQA8gA5V7mESUHDWd4ueGlcKs4AwJL-f6hIA

[leaden blade]

@dry fable Here's a shortened URL of your playground link! You can remove the full link from your message.

sspp0000xx#7906

Playground Link

Preview:```ts
const FooConst = "foo" // without explicit string
const BarConst: string = "bar" // as explicit string

type FooBar = {
foo: typeof FooConst
bar: typeof BarConst
}

let a: FooBar = {
foo: FooConst,
bar: BarConst,
}

let b: FooBar = {
foo: "foo",
...```

You can choose specific lines to embed by selecting them before copying the link.

[lost scroll]

BarConst is a string
so in FooBar, the bar property is also a string (since typeof BarConst)
so in c, bar is also a string, so it accepts zzz

@dry fable

[dry fable]

The 'typeof' is because trying to use it without typeof gives 'FooConst' refers to a value, but is being used as a type here. Did you mean 'typeof FooConst'? - so maybe I just have to solve this in a different way then. 🤔

[unique rune]

https://xyproblem.info you might want to go over that to get a better answer

[lost scroll]

right,
BarConst is a variable, it's the value
typeof BarConst is BarConst's type, so string

[dry fable]

What I am trying to solve was having different classes with "type" in them, where they would be assigned to constants.. but I keep on having to put it in as a value instead ( 'foo' instead of FooType )

[lost scroll]

what you are looking for is called "generics"
https://www.typescriptlang.org/docs/handbook/2/generics.html

[unique rune]

wow how did manage to figure that out lmao

im so lost

[dry fable]

I use plenty of generics, so it's not quite that.. I just want a type to have a fixed value for one property, taken from a constant, instead of having to write it inline as a value

[lost scroll]

having different classes with "type" in them
different classes with a common field

[unique rune]

idk that sounds kinda like discriminated unions to me

[lost scroll]

oh, so typeof then

[unique rune]

if not that, you might want to show a playground of that instead

[dry fable]

typeof works, but it only works if the const is defined without a type. which is what i was trying to understand why

[unique rune]

const a = "foo" has a's type as "foo", not string

with consts, ts gets as narrow as possible

that's what as const does

[dry fable]

right, that probably explains it then - was what I suspected

[unique rune]

you couldve used the playground (or i guess hover over FooConst)

const FooConst = "foo";
//    ^?

!ts

[leaden blade]

const FooConst = "foo";
//    ^? - const FooConst: "foo"

[dry fable]

right, I see that now when I hover - I didn't think of that - thanks

ok well that answers my question then, I'll rephrase and ask later if I can frame it better 🙂

[lost scroll]

if you want to set a constraint without actually changing the type, you might want to use the satisfies operator

https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-9.html#the-satisfies-operator

[dry fable]

yes I noticed that was added in 4.9 - didn't have time to dive into it yet. will have a look

thanks @lost scroll @unique rune