Incremental type narrowing, enabling gradual object initialization

Search Terms

delayed initialization, gradual initialization

Suggestion

What if, on every assignment to either a value or a property, that the type of that value is narrowed? It's probably easiest explained with an example:

Use Cases

It means that this code for gradually initializing an object would just work:

type Person = { firstName: string, surname: string }

function f() : Person {
    const p : Partial<Person> = {}

    p.firstName = ""
    p.surname = ""

    return p; // Error: Type 'Partial<Person>' is not assignable to type 'Person'
}

Currently this code does not type check because the compiler doesn't change the type of p after the assignments to its properties.

My thinking is that, for any value x of type X : { a : unknown },

x.a = "some string"

would result in x having its type narrowed to:

X & { a : string }

This means that the partial initialization example would work. If it works for variables as well as properties then so would this:

function f(mode: "read" | "read/write") {
    // ... Code ...
}

let s: string;

s = "read"

f(s)

Examples

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, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

この issue では実装ファイル、テスト、コンパイラーのエントリーポイントのいずれも指定されていません。まず、提示されたオブジェクト初期化と変数代入の例に対する既存の narrowing の挙動を確認してください。完了とするには、要求された代入の narrowing について合意された設計とテストカバレッジが必要です。