What this snippet does
If 'triangle' is added to the union but not handled in the switch, 's' is no longer of type 'never' in the default case, causing a compile error.
Expected output
Compile Error: Type 'string' is not assignable to type 'never'.
Why practise typing this
Typing control flow & narrowing code builds muscle memory for the symbols and indentation TypeScript uses most, which prose-based typing tests never cover. DevType measures your words per minute and accuracy on this snippet and tracks the individual characters you mistype, so later lessons can target them.
More TypeScript practice
- Using never to ensure exhaustive switchesControl Flow & Narrowing · intermediate
- AssertNever pattern for state machinesControl Flow & Narrowing · intermediate
- Narrowing via switch(true)Control Flow & Narrowing · advanced
- Categorizing values with switch(true)Control Flow & Narrowing · advanced
- Loss of narrowing in non-strict equalityControl Flow & Narrowing · advanced
- Storing a type guard in a variableControl Flow & Narrowing · advanced