How to limit function input to dynamically assigned keys of an object in typescript? - node.js

Question

lately I am trying to create a placeholder object that I will be able to use with typescript.
Idea is to have an empty object and two functions:
one to add new key to a placeholder object with another object as a value ( 'add' function )
and one to get this value by passing a key that already exists in a placeholder ( 'get' function )
I would like typescript to forbid to type keys that already exist in placeholder in 'add' function.
Also I would like to get suggestions while typing key in 'get' function.
Last thing that I would like to achieve is to have type of an object that is returned from 'get' function instead of 'any' or 'object'
Here is the sample code with some basic typing:
let placeholder = {}
function add(key: string, test: object) {
placeholder[ key ] = test
}
function get(key: string ) {
return placeholder[key]
}
add('test1', { val: 1 }) // here 'test1' is ok
add('test1', { val: 2 }) // here 'test1' should rise an error
let t1 = get('') // here 'test1' and should be suggested
t1.val // here t1 should have type { val: number }
So far I have tried using generic types with things like:
function add( key: Omit< string, keyof typeof placeholder >, test: object ) { ... } // it is casting key to properties of string
function get< Key extends keyof typeof placeholder > ( key: Key ) { ... } // it only works with static keys

Answer 1

That is not possible. This would require the type of the object to change, but types are static.
The only thing you could do would be to return an object from your add function with a modified type and then continue calls on that object.
Example of that approach (still has one typing issue i set to ignore):
class Placeholder<T>
{
constructor(private entries: T)
{
}
add<K extends string, V>(key: Exclude<K, keyof T>, value: V): Placeholder<T & { [X in K]: V }> {
const newEntries = { ...this.entries, [key]: value };
// #ts-ignore
return new Placeholder(newEntries);
}
get(key: keyof T) {
return this.entries[key];
}
}
const ph = new Placeholder({})
.add('test1', { val: 1 }) // here 'test1' is ok
.add('test1', { val: 2 }) // here 'test1' should rise an error
let t1 = ph.get('test1') // here 'test1' and should be suggested
t1.val // here t1 should have type { val: number }
[Playground Link]