better type indication for partial/optional type assignment

[loynoir]

Suggestion

🔍 Search Terms

• type indication
• assignment
• Partial<T>
• optional type key?: value

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Better type indication for assignment to Partial<T>, or said key?: value

📃 Motivating Example

Before feat

interface IFoobar {
  a: number
  b: number
  c: number
}

function reproduce() {
  const foobar: Partial<IFoobar> = {}

  foobar.a = 42
  foobar
  // Partial<IFoobar>

  foobar.b = 42
  foobar
  // Partial<IFoobar>

  foobar.c = 42
  foobar
  // Partial<IFoobar>
}

💻 Use Cases

After feat

interface IFoobar {
  a: number
  b: number
  c: number
}

function reproduce() {
  const foobar: Partial<IFoobar> = {}

  foobar.a = 42
  foobar
  // Partial<IFoobar> & Pick<IFoobar, 'a'>

  foobar.b = 42
  foobar
  // Partial<IFoobar> & Pick<IFoobar, 'a' | 'b'>

  foobar.c = 42
  foobar
  // IFoobar
}

[jcalz]

Essentially a duplicate of #42384; narrowing of properties doesn’t currently narrow parent objects.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.