What this snippet does
Because arrays are covariant, pushing a base type into an array of subtypes compiles, but causes a runtime error when subtype methods are called.
Expected output
TypeError: d.bark is not a function
Why practise typing this
Typing advanced generics 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
- Array covariance in TypeScriptAdvanced Generics · advanced
- Function parameter contravarianceAdvanced Generics · advanced
- Defining a linked list node typeAdvanced Generics · advanced
- Typing a deeply nested JSON structureAdvanced Generics · advanced
- Infinite recursion in conditional typesAdvanced Generics · advanced
- Inferring only the first type argumentAdvanced Generics · advanced