Generic Constraints

Sometimes you want generics, but with rules.

    ## Example: must have length
    
    ```ts
    function logLength<T extends { length: number }>(val: T): T {
      console.log(val.length);
      return val;
    }
    
    logLength("hello");
    logLength([1, 2, 3]);
    // logLength(10); ❌
    ```
    
    ## With interfaces
    
    ```ts
    interface HasId {
      id: number;
    }
    
    function printId<T extends HasId>(obj: T) {
      console.log(obj.id);
    }
    ```
    
    ## Why constraints?
    So TypeScript knows what properties exist.
    
    ## Graph
    ```mermaid
    flowchart TD
      A[T] --> B{extends rule}
      B --> C[Allowed]
      B --> D[Rejected]
    ```
    
    ## Remember
    - Use extends to limit generics
    - Keeps flexibility with safety