Typescript Stephen Grider May 2026

(This is deliberately weak.) interface AddAction type: 'add'; payload: number;

When you use if (isAddAction(action)) , TypeScript narrows the type inside the block. Grider calls this "teaching the compiler your business logic." typescript stephen grider

function isAddAction(action: CounterAction): action is AddAction return action.type === 'add'; (This is deliberately weak

Enter .

After his course, you will open a tsconfig.json and see not a wall of noise, but a safety checklist. You will write functions where the type signature tells a story: "Given this, I guarantee that." You will refactor a 500-line JavaScript file into a 200-line TypeScript file, removing entire categories of bugs. You will write functions where the type signature

Stephen Grider does not promise you will become a TypeScript contributor to the compiler. He promises you will never again push a bug where undefined is not a function at 2 AM.