TypeScript — Type Inference

Type guards

function isString(value: unknown): value is string {
  return typeof value === 'string';
}

function process(value: string | number) {
  if (isString(value)) {
    console.log(value.toUpperCase());
  } else {
    console.log(value.toFixed(2));
  }
}

Discriminated unions

type Shape =
  | { kind: 'circle'; radius: number }
  | { kind: 'rectangle'; width: number; height: number };

function area(shape: Shape): number {
  switch (shape.kind) {
    case 'circle':
      return Math.PI * shape.radius ** 2;
    case 'rectangle':
      return shape.width * shape.height;
  }
}

Exhaustive checking

function assertNever(value: never): never {
  throw new Error(`Unexpected value: ${value}`);
}

function processShape(shape: Shape) {
  switch (shape.kind) {
    case 'circle': return Math.PI * shape.radius ** 2;
    case 'rectangle': return shape.width * shape.height;
    default: return assertNever(shape);
  }
}

Mini Practice

Write TypeScript code that:

1. Creates a type guard function
2. Uses discriminated unions
3. Demonstrates exhaustive checking
4. Narrows types with typeof and instanceof

Up Next

In the next lesson, you'll learn about Error Handling — Result types and error patterns.