I'm having trouble setting a value for an object with a generic type

I am attempting to set a value for the property of an object with generic typing passed into a function. The structure of the object is not known beforehand, and the function receives the property name dynamically as a string argument. TypeScript is generating the error "Type 'string' cannot be used to index type 'T'."

My TypeScript code looks like this:

interface Value {
  [key: string]: any
}

interface SomeInterface<T> {
  key: string,
  value: T
}

function someFunction<T extends Value>({ key, value }: SomeInterface<T>) {
  value[key] = somevalue;
}

The line where I assign somevalue to the value triggers the error "Type 'string' cannot be used to index type 'T'", despite having created the index signature in the Value interface.

What could be the reason behind this issue?

Answer №1

An issue arises when you try to invoke someFunction with a more specific type of Value.

someFunction<{'specificKey': any}>(...);

As a result, the use of string as an index for value becomes invalid. Only specificKey can now serve as a valid index. To solve this, you can modify SomeInterface in a way that ensures it always contains a key suitable for indexing:

interface SomeInterface<T> {
  key: keyof T,
  value: T
}

See full example

Answer №2

When you define the generic type key for the specific object you're working with, it allows you to have clear types for both the key and its corresponding value.

interface Data {
  [id: string]: any
}

function assignValue<K extends keyof Data>({ id, content }: {id: K, content: Data[K]}) {
  content[id] = content;
}

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