unexpected behavior when use union type

[MaoScut]

TypeScript Version: 3.1.4

Search Terms:
union type
Code

export type basetype = {
  id: string;
}
export type supertype1 = basetype & {}
export type supertype2 = basetype & {
  key1: any;
  key2: any;
}

function myfun(p: supertype2 | supertype1) {
  return 
}

myfun( {
  id: '',
  key1: '',
})

Expected behavior:
{
id: '',
key1: '',
} is not supertype1 or supertype2, when it is used to be a parameter of myfun, typescript should report an error.
Actual behavior:
there is no error.

[j-oliveras]

The myfun parameter is assignable to supertype1 (that is basically the same as basetype). Remember that the type are checked structurally.

Maybe that you expect is a exact type. See discussions about that: #202, #12936 and PR #28749.

[ahejlsberg]

Write it like this instead:

export type basetype = { id: string }

export type supertype1 = basetype & { key1?: undefined, key2?: undefined }
export type supertype2 = basetype & { key1: any, key2: any; }

function myfun(p: supertype2 | supertype1) {}

myfun({ id: '' });  // Ok
myfun({ id: '', key1: '' });  // Error
myfun({ id: '', key1: '', key2: '' });  // Ok

[MaoScut]

The myfun parameter is assignable to supertype1 (that is basically the same as basetype). Remember that the type are checked structurally.

Maybe that you expect is a exact type. See discussions about that: #202, #12936 and PR #28749.

I know structural typing, so I pass an object literal instead of a variable to myfun. And Exact type is what I need. Thank you.

[MaoScut]

Write it like this instead:

export type basetype = { id: string }

export type supertype1 = basetype & { key1?: undefined, key2?: undefined }
export type supertype2 = basetype & { key1: any, key2: any; }

function myfun(p: supertype2 | supertype1) {}

myfun({ id: '' });  // Ok
myfun({ id: '', key1: '' });  // Error
myfun({ id: '', key1: '', key2: '' });  // Ok

Nice. It work for me. Thank you.
interface { key1?: undefined}
describes key1 is a forbidden property. it's better to add this syntax to TypeScript documents.(handbook --> interface --> Optional Properties)

[typescript-bot]

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