Constraining Generics with Extends
When working with generics in TypeScript, there are times when you need to ensure that a generic type meets certain requirements. You can achieve this by using the extends keyword to constrain the generic type. This approach allows you to specify that a generic type parameter must have a particular structure or inherit from a specific type. By constraining generics, you make your code more robust and help prevent runtime errors by catching type mismatches at compile time.
1234567function logLength<T extends { length: number }>(item: T): void { console.log(`Length is: ${item.length}`); } logLength("hello"); // Output: Length is: 5 logLength([1, 2, 3]); // Output: Length is: 3 // logLength(42); // Error: Argument of type 'number' is not assignable to parameter of type '{ length: number; }'
By using T extends { length: number }, you ensure that the function only accepts arguments that have a length property, such as strings and arrays. This constraint means that if you try to call logLength with a value that does not have a length property, TypeScript will display a compile-time error. This improves type safety by preventing accidental misuse of the function. Constraints like these are especially useful when you want to write generic code that works with a family of related types, but not with every possible type.
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Can you explain how to use other constraints with generics in TypeScript?
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When working with generics in TypeScript, there are times when you need to ensure that a generic type meets certain requirements. You can achieve this by using the extends keyword to constrain the generic type. This approach allows you to specify that a generic type parameter must have a particular structure or inherit from a specific type. By constraining generics, you make your code more robust and help prevent runtime errors by catching type mismatches at compile time.
1234567function logLength<T extends { length: number }>(item: T): void { console.log(`Length is: ${item.length}`); } logLength("hello"); // Output: Length is: 5 logLength([1, 2, 3]); // Output: Length is: 3 // logLength(42); // Error: Argument of type 'number' is not assignable to parameter of type '{ length: number; }'
By using T extends { length: number }, you ensure that the function only accepts arguments that have a length property, such as strings and arrays. This constraint means that if you try to call logLength with a value that does not have a length property, TypeScript will display a compile-time error. This improves type safety by preventing accidental misuse of the function. Constraints like these are especially useful when you want to write generic code that works with a family of related types, but not with every possible type.
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