What this snippet does
Removing optionality via mapped types is a compile-time illusion; runtime undefined values still cause crashes if cast.
Expected output
TypeError: Cannot read properties of undefined (reading 'toUpperCase')
Why practise typing this
Typing mapped types 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
- Getting keys of an object as a unionMapped Types · intermediate
- Restricting function parameters to object keysMapped Types · intermediate
- Using keyof on primitive typesMapped Types · intermediate
- Creating a custom readonly mapped typeMapped Types · intermediate
- Creating a nullable version of an interfaceMapped Types · intermediate
- Mapped types altering value types incorrectlyMapped Types · intermediate