Value declared with interface not assignable to Record<string, unknown>

[MarkDuckworth]

🔎 Search Terms

"T extends Record<string, unknown>", "interface extends Record<string, unknown>", "interface not assignable to Record<string, unknown>

🕗 Version & Regression Information

• This is the behavior in every version I tried (^3.x, ^4.x, ^5.x), and I reviewed the FAQ for entries about types vs interfaces, conditional types, and unknown type

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.7.2&ssl=8&ssc=38&pln=8&pc=1#code/PTAEBUFMGcBcEsB2BzUsDuB7U8AmlEEBjAQwBtRoALEgBxgC5QBZAT3FftBMVxdYCShSACcAZiSKQAULE6R+HLgF5QAb1BjMmJgCNtZSD1ABfaUliiJU-kMvjJCjVp2h9mQ8bPSQEGLG5oaHhkRCRUWGwAJUgiTBFcAB44EXCAGlAAV0QAa0RMdEQAPmk4xDhuJjY7K0dQVWdtJlgRTIUTAG4fMAAxEngyaCYlBQByauEHKVGcaFB8gJIgkMQSXUM0bDkuUZi4hOSW9Kzc-MKi0dLMcoDdJj34pJTj7LyC4vruLqub0CIq9jyT6NVwtNqmLplCq4e6xR6HVIoDKvM4fVREb6+KBwcIQUCQAAell4cweB2eSJOb3OsiBAmgZNwAGUjigAKqnd6JcBFT7gfFEgi4Ulw8ms5DIznnUAAflAo1YMBmTFG+VGkOuFUgTHpjJZiOQHOpiESbBGvNUCqVXV8AEE5iRQCIYJkyLAMhN7NYFLhMDB5pgAoTiXxGQiXlLij8KmIdQzRczxUbUabBJNvRb5WrvtJc7nfMpC4XQLbcLh4AhruQ0P45rpIGQCvUi8powFkHoDEZEAKQ3MUe9ZfLFdBlVnMDNLSP1W3QFQATUppAANqjFyjAC6vaF-cjQ6to9AKrVnwPM+6fhxKDxwZ3oDDFIl3EQrBK2wUuoT+vCtpf3Mz-K3iS94JuGlI8K++7Tke44zlCATwHGerir+rCpuap7ThqvwAFZIV+KF-p6tRSJmZ4dEAA

💻 Code

// Testing two identical shapes: MyType and MyInterface
type MyType = { foo: boolean }
interface MyInterface { foo: boolean }

// Test assigning to Record<string, unknown>
const a: MyInterface = { foo: true };
// Fails: Type 'MyInterface' is not assignable to type 'Record<string, unknown>'
const b: Record<string, unknown> = a;

// This is fine when the declaration uses MyType
const c: MyType = { foo: true };
const d: Record<string, unknown> = c;

// Testing T extends Record<string, unknown>
type IsRecordStringUnknown<T> = T extends Record<string, unknown> ? 'yes' : 'no';
const e: IsRecordStringUnknown<MyType> = 'yes';
// Similarly, MyInterface does not extend Record<string, unknown>
const f: IsRecordStringUnknown<MyInterface> = 'no';

🙁 Actual behavior

• Type MyInterface is not assignable to type Record<string, unknown>
• MyInterface does not extend Record<string, unknown>.

🙂 Expected behavior

MyInterface is assignable to Record<string, unknown>, so the following code should compile:

const a: MyInterface = { foo: true };
const b: Record<string, unknown> = a;

MyInterface extends Record<string, unknown>, so the following code should compile:

const d: IsRecordStringUnknown<MyInterface> = 'yes';

Additional information about the issue

The most relevant documentation I could find related to this:

// Anything but primitive assignable to { [x: string]: unknown }
function f24(x: { [x: string]: unknown }) {
  x = {};
  x = { a: 5 };
  x = [1, 2, 3];
  x = 123; // Error
}

source

[MartinJohns]

This is working as intended. interface and type are not interchangeably.

Duplicate of #42825. See also: #15300 (comment)

[typescript-bot]

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