Allowing union types to work on control flow analysis

[max-hk]

Suggestion

🔍 Search Terms

• Control flow analysis
• Union types

✅ 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

It would be better if we allow union types to work on control flow analysis

📃 Motivating Example

type Shape =
    | { kind: "circle", radius: number }
    | { kind: "square" | 'rectangle', sideLength: number };

function area(shape: Shape): number {
    if (shape.kind === "square" || shape.kind === "rectangle") {
        // We know we have a square/rectangle here!
        return shape.sideLength ** 2;
    }
    else {
        // We know we 're left with a circle here!
        // But TypeScript failed to recognize it
        return Math.PI * shape.radius ** 2;
    }
}

💻 Use Cases

[MartinJohns]

Playground link

[MartinJohns]

This sounds like a duplicate of #31404. See this comment for a workaround.

[andrewbranch]

I retitled that issue since it wasn't actually related to undefined.

[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.