Cannot safely, generically add a property to a record

[SLaks]

TypeScript Version: v4.0.0-dev.20200525, 3.9.2

Search Terms: object intersection record

Code

function add<TShape, TName extends string, TValue>(
    obj: TShape, name: TName, value: TValue): TShape & Record<TName, TValue> {
  return {...obj, [name]: value};
}

Expected behavior:

No compiler errors

Actual behavior:

Type 'TShape & { [x: string]: TValue; }' is not assignable to type 'TShape & Record<TName, TValue>'. Type 'TShape & { [x: string]: TValue; }' is not assignable to type 'Record<TName, TValue>'.

Playground Link

Additional attempts:

I couldn't find any way to make this work without a cast; no matter what I tried, the property [name]: value is inferred as [x: string]: TValue, ignoring TName. Other attempt:

  const addition: {[name in TName]: TValue} = {[name]: value};
  return {...obj, ...addition};

Type '{ [x: string]: TValue; }' is not assignable to type '{ [name in TName]: TValue; }'.

Playground Link

Related Issues: Possibly #18538

[SLaks]

On further thought, this wouldn't actually be safe, because TName could also be a union:

const bad = add<{x: 1}, 'a'|'b', 'Oops'>({x: 1}, 'a', 'Oops');
bad.b;

Playground Link

I guess this is a feature request then, to have some way of accepting a single property name as a parameter so this can be safe. I don't have a specific design offhand.

[SLaks]

Motivation

I want to create a hierachical builder that accepts property names as strings and builds an object with those properties.

For (a simplified) example:

Playground Link

[RyanCavanaugh]

I can't tell what you're trying to do here - please boil this down to something more actionable if you'd like us to follow up more. Thanks!

[SLaks]

General feature request

Have a type-safe way to accept a string parameter and use it as a property name. In other words, a way to write the following code without the cast:

function add<TObject, TName extends string, TValue>(
    obj: TObject, name: TName, value: TValue) {
  return {...obj, [name]: value} as TObject & {[name in TName]: TValue};
}

This may require some way of accepting a single property name as a parameter (as opposed to 'a' | 'b') so this can be safe.

[RyanCavanaugh]

Got it - this would be a duplicate (or at least dependent on) of #27808 then