TypeScript in Depth · Generics · Lesson 10 of 16
Constraints on Generics
Concept
Constraints on Generics
The extends keyword lets you restrict a generic type to only types that have certain properties.
By the end of this lesson
- Explain the core idea behind Constraints on Generics.
- Predict output before running a change.
- Test one realistic and one unusual input.
- Use the result to make the next decision.
How to study this page
- 1. Read one concept.
- 2. Change the example.
- 3. Run, compare, and explain.
- 4. Complete the challenge below.
ExampleRunnable
function getLength<T extends { length: number }>(item: T): number {
return item.length;
}Self-check before continuing
Without looking at the example, describe what changes when you modify one input in Constraints on Generics. Then reopen the editor and prove your explanation with a small test.
You are ready to continue when you can predict, test, and explain the result.
Your turn
Explain why getLength([1,2,3]) works but getLength(42) would fail to compile.
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Console