Skip to content

Instantly share code, notes, and snippets.

@pesterhazy

pesterhazy/typescript-interface-wat.ts

Last active August 12, 2025 11:03
Show Gist options
  • Save pesterhazy/444e51b4b52a06661c8402ff9ee4959c to your computer and use it in GitHub Desktop.
Save pesterhazy/444e51b4b52a06661c8402ff9ee4959c to your computer and use it in GitHub Desktop.
Download ZIP

Revisions

  1. pesterhazy revised this gist Aug 12, 2025. No changes.
  2. pesterhazy revised this gist Aug 7, 2025. 1 changed file with 5 additions and 0 deletions.
    5 changes: 5 additions & 0 deletions typescript-interface-wat.ts
    View file
    Original file line number Diff line number Diff line change
    @@ -26,3 +26,8 @@ test.only("experiment 2", async () => {
    experiment = new Experiment();
    experiment.foo({}); // oops, we called foo without passing bar, causing a runtime error
    });

    /*
    How do I change the code so that I can be sure that every class that implements IExperiment
    is safe to use with through the interface?
    */
  3. pesterhazy revised this gist Aug 7, 2025. 1 changed file with 5 additions and 2 deletions.
    7 changes: 5 additions & 2 deletions typescript-interface-wat.ts
    View file
    Original file line number Diff line number Diff line change
    @@ -6,9 +6,12 @@ interface IExperiment {
    }

    class Experiment implements IExperiment {
    // Why does Experiment implement IExperiment, even though IExperiment.foo doesn't
    // guarantee that opts.bar is defined?
    // Wat?!
    // Why is Experiment allowed to implement IExperiment (the typechecker doesn't complain)
    // even though IExperiment.foo doesn't guarantee that opts.bar is defined?

    foo(opts: { bar: string }): void {
    // foo needs opts.bar to be there
    assert.ok(opts.bar);
    }
    }
  4. pesterhazy created this gist Aug 7, 2025.
    25 changes: 25 additions & 0 deletions typescript-interface-wat.ts
    View file
    Original file line number Diff line number Diff line change
    @@ -0,0 +1,25 @@
    import test from "node:test";
    import assert from "node:assert";

    interface IExperiment {
    foo(opts: {}): void;
    }

    class Experiment implements IExperiment {
    // Why does Experiment implement IExperiment, even though IExperiment.foo doesn't
    // guarantee that opts.bar is defined?
    foo(opts: { bar: string }): void {
    assert.ok(opts.bar);
    }
    }

    test.only("experiment", async () => {
    const experiment = new Experiment();
    experiment.foo({ bar: "baz" });
    });

    test.only("experiment 2", async () => {
    let experiment: IExperiment;
    experiment = new Experiment();
    experiment.foo({}); // oops, we called foo without passing bar, causing a runtime error
    });