Roadmap

    3 - Drawbacks of Type Predicates

    Drawbacks of Type Predicates

    While type predicates are very useful, they have some drawbacks:

    1. Runtime Overhead: Type predicates add runtime checks to your code. For complex types or frequent checks, this can slow down performance.
    2. Limited by JavaScript Capabilities: Type predicates can only check what JavaScript can do at runtime. They can't enforce more complex TypeScript-only types, like interfaces with methods or generic types.
    3. Maintainability: We can cause problems if the type definitions are updated but the type predicates are not.
    4. False Security: If not written correctly, type predicates can give a false sense of security, leading to potential bugs if the type checks are not thorough.

    Here's an example of how a poorly written type predicate can create a false sense of security, leading to potential bugs:

    ts
    interface Button {
      value: "Login";
      onClick: () => void;
    }
     
    interface Input {
      value: "Login";
      name: "username";
    }
     
    type Field = Button | Input;
     
    function isButton(element: Field): element is Button {
      return (element as Button).value === "Login";
    }
     
    function handleField(element: Field) {
      if (isButton(element)) {
        // TypeScript now believes 'element' is a Button
        element.onClick(); // This line is considered safe by TypeScript
      } else {
        console.log(element.name); // This line is considered safe by TypeScript
      }
    }
     
    // Simulated events
    const unknownField: Field = { value: "Login", name: "username" };
    handleField(unknownField);

    Here is a function with no Typescript errors, but an improperly written type predicate produces errors at runtime:

    [ERR]: element.onClick is not a function

    The problem is that it assumes that the presence of value: "Login" guarantees that element is indeed a Button. The check only looks at the value property without verifying the presence of the onclick method.

    Because the predicate is incomplete (only checking the value property), it can lead to assumptions that aren't true, potentially causing bugs at runtime.

    In most cases