With TypeScript 2.0, you can add a tag to union types and use these as type guards. That feature is called: discriminated union types. This sounds very difficult, but in practice it is very easy. The following example demonstrates it:
interface Circle {
type: "circle";
radius: number;
}
interface Square {
type: "square";
size: number;
}
type Shape = Circle | Square;
function area(shape: Shape) {
if (shape.type === "circle") {
return shape.radius * shape.radius * Math.PI;
} else {
return shape.size * shape.size;
}
}
The condition in the if statements works as a type guard. It narrows the type of shape to circle in the true branch and square in the false branch.
To use this feature, you must create a union type of which all elements have a property with a string value. You can then compare that property with a string literal and use that as a type guard...