Have you ever used discriminant unions and type guards? What built-in type guards do you know?

Discriminated unions and type guards are powerful TypeScript tools for working with union types, allowing safe determination of a specific type within a union.

Discriminated unions are type unions where each type has a common literal key (for example, a type field), which can be used to unambiguously identify the specific type.

Type guards are functions or constructs that check the type of a variable at runtime and narrow the type for the compiler.

Built-in type guards in TypeScript include:

• typeof — for primitives (string, number, boolean)
• instanceof — to check class membership
• in — to check for the presence of a property in an object

Example of a discriminated union and type guard:

interface Square {
  kind: 'square';
  size: number;
}

interface Circle {
  kind: 'circle';
  radius: number;
}

type Shape = Square | Circle;

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

Here, kind is the discriminator, and switch acts as a type guard.

Using built-in type guards:

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

if (isString(someVar)) {
  // here, someVar is of type string
}