valid generic manipulation helping

[foggy latch]

I cant find a proper way to define a class with generics so that i can implement method add as in the ts playground link below.

class X: I may understand that it may be cause index 'a' might not be a valid index?
class Y: dont really know
class Z: all good
class G: just another try to have it working but it says that "number is not assignable"?

[hushed fractal]

andreaelektronvolt#0

Playground Link

Preview:```ts
class X<T extends Record<string, unknown> {

constructor(public val: T) {

}
add() {
this.val['a'] = 1;
}
}

class Y<T extends {[k in (string | 'a')]: any}> {
constructor(public val: T) {

}
add() {
this.val['a'] = 1;
}
}

class Z<T extends {a: any}> {
...```

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[ebon grail]

What is it that you are trying to do?

Is add supposed to always set val.a to 1?

[foggy latch]

my current real class is similar to G

i would expect G.add to be working

as m should be a valid key in T

no?

[hushed fractal]

nonspicyburrito#0

Playground Link

Preview:```ts
class G<T extends Record<string, number>> {
constructor(public val: T) {}

add(m: keyof T) {
this.val[m] = 1
}
}

const g = new G({foo: 42 as const})
// ^?

g.add("foo")

g.val.foo
// ^?```

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[ebon grail]

Here's a demonstration why that's not allowed.

g is G<{ foo: 42 }>, but after calling g.add('foo') the value gets changed to 1, which no longer matches the type.

[foggy latch]

hmm

now i get it

[ebon grail]

If you instead rewrite it into:

class G<K extends string> {
    constructor(public val: Record<K, number>) {}

    add(m: K) {
        this.val[m] = 1
    }
}

Then it will work.

[foggy latch]

gotcha