I have a function which looks as so:

interface Message {
  id: string;
  content: string;
  created_at: Date;
  updated_at: Date;
  priority: 'low' | 'medium' | 'high'
}

function merger<T extends Message, K extends keyof T>(key: K, value: T[K], data: T) {
  if (key === 'content') {
    return `${data[key]}${value}`
  }
  return value;
}

// ...
const merge = mergeWith(merge, current, incoming);

The problem, however, is typescript is not able to infer the type correctly from the key. I would expect that if K is content, it would be able to infer T['content'] would be of type string

Is there a way to solve this?

Here’s what I tried. I don’t really like it

interface Message {
  id: string;
  content: string;
  created_at: Date;
  updated_at: Date;
  priority: 'low' | 'medium' | 'high'
}

function isContent<T extends Message, K extends keyof T>(key: K, data: T): T[K] is string {
  return key === 'content';
}

function merger<T extends Message, K extends keyof T>(key: K, value: T[K], data: T) {
  if (isContent(key, value)) {
    return `${data[key]}${value}`
  }
  return value;
}

// ...
const merge = mergeWith(merge, current, incoming);