TS Index Signature


## TS Dynamic Attribute
index type
```typeclass Foo {    hello = "hello";    world = 1234;    // This is an index signature:    [propName: string]: string | number | undefined;}let instance = new Foo();// Valid assigmentinstance["whatever"] = 42;// Has type 'string | number | undefined'.let x = instance["something"];```


### Common errors

[An index signature parameter type cannot be a literal type or generic type. Consider using a mapped object type instead](https://texonom.com/an-index-signature-parameter-type-cannot-be-a-literal-type-or-generic-type-consider-using-a-mapped-object-type-instead-55257551021a4727987b9250693c71a2)

```typeclass Foo {    static hello = "hello";    static world = 1234;    static [propName: string]: string | number | undefined;}// Valid.Foo["whatever"] = 42;// Has type 'string | number | undefined'let x = Foo["something"];```




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