[TIL] TypeScript type predicates

Type narrowing is the process by which a type is refined to a more specific type.

The TypeScript compiler does its best to narrow types using the existing JavaScript code.

TIL about TypeScript type predicates, a way to be more explicit and have better control over type inference.

First, we define a function whose return type is a type predicate:

function isAudio(content: Audio | Text): content is Audio {
  return (content as Audio).duration !== undefined;
}

We can now use isAudio to narrow the type of a variable we're working with, allowing TypeScript from that point on to know the more specific type:

let content = getContent();

if (isAudio(content)) {
  content.play();
} else {
  content.render();
}

Resources:

• Narrowing - TypeScript documentation
• Type predicates - TypeScript documentation