What this snippet does
TypeScript trusts the type predicate blindly, leading to runtime crashes if the logic is incorrect.
Expected output
TypeError: val.toUpperCase is not a function
Why practise typing this
Typing type guards 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
- Narrowing a union with typeofType Guards · beginner
- Processing dynamic input based on typeofType Guards · beginner
- Using incorrect casing in typeof checksType Guards · beginner
- Narrowing class instances with instanceofType Guards · beginner
- Handling specific error subclassesType Guards · beginner
- Using instanceof on primitive stringsType Guards · beginner