Uncaught Type assertion between Interface and Class Types can lead to runtime errors

[Hedayet]

🔎 Search Terms

interface assertion

🕗 Version & Regression Information

• This is the behavior in every version I tried, and related issue exists in the FAQ - (https://www.typescriptlang.org/play/?#code/JYOwLgpgTgZghgYwgAgJIDED2nkG8BQAvvvggDZwDOlyWOwAtgA5kQMTg0bZ77LKU4ATwASEMmRwBeZAAoAlMikA+Xv34JMISplYA6SQHNZAcgAW4ycgDumKGQAmJ+X2SEA3ERKbtYZDGwALjQ6JTwPby1KPwAjOCgwgJwqWmx3ZAB6DOQIYDALBPzgGkozTABXR2QYlFAANzgyYAdSKN0IA0xjOKg9QVFLTEV+LOQ7ZCKSssqHEBNYlHKQBwgYUAgHIA)

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/JYOwLgpgTgZghgYwgAgJIDED2nkG8C+AUIQgDZwDOFyWOwAtgA6kT0TjUbZ6HLIKYQFMFACuCMJigAKRqIBGpYAmQg4bAFzJhUUAHMAlHiJ8KcAJ4AJCKVI4AvMmlH7APh58+AoZhYA6Oz1pAHIACxs7ZAB3KVIAE2CDAG5eZHwUohJBYWQYbC0uB2MUrKEwXOwAeXkAKwgJLVpkRzycShpsJOQAem7kMFDgagpQzFF45HkUUAA3OCU40opfCADMINbquok-MysIzGTPXuRREDiIGFAIRe9l-0DpTdr6sD81NiPPY76zi6uQDdCEA

💻 Code

interface IFoo {}

class Foo implements IFoo {
  constructor(public name: string) {}
  sayHello = () => {
    console.log('hello world');
  };
}

const foo: IFoo = {};

const fooObject: Foo = foo as Foo; // this should be invalid
console.log(fooObject.sayHello);   // undefined
console.log(fooObject.name);       // undefined

🙁 Actual behavior

Structural type checking might be confusing and leaves space for unintended runtime errors.

1. Define an interface IFoo
2. created a class Foo that implements IFoo and adds a method (sayHello)
3. created an object literal (foo) conforming to the IFoo interface
4. used a type assertion to cast the object literal to the Foo type (fooObject)
5. attempted to call the sayHello method and use the name field on the type-asserted object (fooObject)

What Went Wrong:

1. Code complies successfully
2. Code will fail during runtime as sayHello and name are undefined on the type-asserted object (fooObject)

🙂 Expected behavior

The type assertion to cast foo as Foo should fail

Additional information about the issue

I understand TypeScript's structural typing system does not check whether the object was created by which class constructor. But in my opinion this is a dangerous gap (even if intentional)

[bgenia]

Type assertions are unsafe by design, as they allow to cast less specific types to more specific types. You can check out #56235 for safe type assertions proposal and its current alternatives.

[typescript-bot]

This issue has been marked as "Working as Intended" and has seen no recent activity. It has been automatically closed for house-keeping purposes.