TypeScript keyof and typeof

These operators help you create types from values.

    ## typeof
    
    ```ts
    const user = { id: 1, name: "Tom" };
    type User = typeof user;
    ```
    
    Now User is { id: number; name: string }
    
    ## keyof
    
    ```ts
    type UserKeys = keyof User; // "id" | "name"
    ```
    
    ## Use together
    
    ```ts
    function getProp<T, K extends keyof T>(obj: T, key: K) {
      return obj[key];
    }
    
    getProp(user, "name");
    // getProp(user, "age"); ❌
    ```
    
    ## Graph
    ```mermaid
    flowchart LR
      A[Object] --> B[typeof]
      B --> C[Type]
      C --> D[keyof]
      D --> E[Union of keys]
    ```
    
    ## Remember
    - typeof turns value into type
    - keyof gives safe property names