Need help on Generic extend string purpose

Hi need help to understand following typescript

1. Generic K extends string

2. If K is restricted to string type then how can we iterate using key in K

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   type NewMappedTypes<K extends string> = { [key in K]: boolean } // 1
   //2. if Generic is restricted to string type then how can we iterate over keys
   function composeTypedMap<K extends string>(k: NewMappedTypes<K>) {
       return k;
   }
   
   const t = composeTypedMap({ 'a': true ,'b':false});
   console.log(t)
   

PlaygroundLink

>Solution :

K extends string is a generic constraint. You’re right in thinking that K must be assignable to string.

However (while the syntax doesn’t make it obvious) it doesn’t mean just "one" string can be used for K: you can supply a union of string literal types, which will be used in the type mapping operation of that type utility.

Here’s another example to help clarify what’s happening:

TS Playground

// Maps the members (constituents) of the string union to keys in the object:
type StringUnionToKeysInObjectOfBooleanValues<U extends string> = {
  [Str in U]: boolean;
};

type ExampleStringLiterals = 'foo' | 'bar' | 'baz';

type ExampleObject = StringUnionToKeysInObjectOfBooleanValues<ExampleStringLiterals>;
   //^? Looks like:
/*

type ExampleObject = {
    foo: boolean;
    bar: boolean;
    baz: boolean;
}

*/

type ObjectFromJustString = StringUnionToKeysInObjectOfBooleanValues<string>;
   //^? Looks like:
/*

type ObjectFromJustString = {
    [x: string]: boolean;
}

*/

In the code you’ve shown, the compiler is using type inference to infer the types of the keys in the object literal that you provide as the argument to the function:

{ 'a': true ,'b':false}

They are inferred as the string union "a" | "b" and become the actual type used in place of the generic type parameter K.