Type guard should infer the type of parent object when applied on a property

Suggestion

🔍 Search Terms

Type guard, parent object, infer, inference

✅ 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

Typescript should be able to infer the type of an object, if a type guard was checked on a property of the said object. Currently, Typescript does correctly infer the type of the property, but not its parent object.

📃 Motivating Example

The following example is very common in data oriented design.

interface Type1 { list: string[] }

interface Type2 { list: { [key: string]: string } }

declare const obj: Type1 | Type2;

if(Array.isArray(obj.list)) {
    const list: string[] = obj.list; // WORKS
    const map: { [key: string]: string } = obj.list; // ERROR, as expected
    const objCasted: Type1 = obj; // ERROR, unexpectedly
} else {
    const map: { [key: string]: string } = obj.list; // WORKS
    const list: string[] = obj.list; // ERROR, as expected
    const objCasted: Type2 = obj; // ERROR, unexpectedly
}

The following example works and that is good because it is an equally common case in this type of design.

interface Type3 { type: 'type3', data: boolean }

interface Type4 { type: 'type4', data: string }

declare const obj2: Type3 | Type4;

if(obj2.type === 'type3') {
    const objCasted: Type3 = obj2; // WORKS
} else {
    const objCasted: Type4 = obj2; // WORKS
}

So I believe the type guards should work the same way. As far as I see, this does not cause any inconsistency in the language or the type system. It is an improvement without any downsides.

💻 Use Cases

See the full example.

[RyanCavanaugh]

I want to take another look at this; seems like we might have the right machinery in place now

[bartenra]

Here is a workaround for my use case:

function assertField<T extends { [field in Field]?: T[field] }, Field extends keyof T & string,>(
  obj: T,
  field: Field
): obj is T & { [field in Field]: NonNullable<T[field]> } {
  if (!obj[field]) false;
  return true;
}

type Slot = { id?: string }

function needsId(slot: { id: string }) {
}

function test(slot: Slot) {

  if (slot.id) {
    slot.id;  // string | undefined

    // needsId(slot)  ERROR
  }

  if (assertField(slot, 'id')) {
    slot.id;  // string

    needsId(slot);  // WORKS
  }
}

Playground