TypeScript allows assignment to a union type with missing required properties

[avivdy]

🔎 Search Terms

When using a union type that includes an extended interface with required properties, TypeScript allows an object to be assigned to the union type even if it has some but not all of the properties of one of the union members. This leads to a runtime error when accessing missing properties.

🕗 Version & Regression Information

v5.7.3

⏯ Playground Link

https://www.typescriptlang.org/es/play/?ssl=21&ssc=1&pln=1&pc=1#code/JYOwLgpgTgZghgYwgAgHJwLYoN4ChnIiYQBcyAzmFKAOYDcuAvrrqJLIigArTkD2IAOrAwACwBCwKGOQQAHpBAATcmmLI8BJXEgB5GJOmiyAER0QGBAA4AbTvsNiylaiHpMWYAJ5XuvAcgAvGpYyAA+yDxQ-EIiElJiDLgIApTIcABuwBlkUTFBGoTEZADkAIJZGSUANMi29gYJxsglACoAMgBqJciMdMgA9ANofLJQUHxQyKLQECwIUBDmAKrk0AAUmdkAlEkwAK4gCGDAAQtLkKsbvtECuf4g2xr4yMAwyOsl9UgOTT2gdQeT00BEBtxAADptHpGkYITQIGAzF51rtBsNWqJgKoAO7AGw2ZAIOD7NbpZBQQ4nULQCZQF7MZhAA

💻 Code

interface Name {
  name: string;
}

interface PersonWithBirth extends Name {
  dateOfBirth: Date;
  placeOfBirth: string;
}

type Person = Name | PersonWithBirth;

const aviv: Person = { name: 'Aviv', placeOfBirth: 'TLV' }; // No error here

createUser(aviv);

function createUser(person: Person) {
  if ('placeOfBirth' in person) {
    person.dateOfBirth.getDay(); // This will cause a runtime error
  }
}

🙁 Actual behavior

TypeScript does not throw an error at the assignment of aviv, leading to a possible runtime error when accessing dateOfBirth.

🙂 Expected behavior

TypeScript should throw a type error at the assignment of aviv because it includes placeOfBirth, which is only defined in PersonWithBirth, but it is missing dateOfBirth, which is required by PersonWithBirth.

Additional information about the issue

No response

[MartinJohns]

This is working as intended. Excess property checks don't happen for union types (see #20863) and your object is assignable to { name: string }.

Also, the in operator is (intentionally) unsafe (see #34975), assuming here your object is a PersonWithBirth when it's not.

[avivdy]

thanks! @MartinJohns