Introduction

In a TypeScript program, a variable can move from a less precise type to a more precise type. This process is called type narrowing.

Consider the following function:

Copy
type Animal = {
  name: string;
  legs?: number;
};
function addLeg(animal: Animal) {
  animal.legs = animal.legs + 1; // 💥 - Object is possibly 'undefined'
}

TypeScript raises a type error because the legs property could be undefined, and it doesn't make sense to add 1 to undefined.

To resolve this type error, we need to narrow the type of the legs property to number. i.e. we need to remove undefined from its type.

In this module we will learn different ways in which we can narrow types to prevent type errors. We will cover:

• Using type assertions
• Using the non-null assertion operator
• Using a typeof type guard
• Using an instanceof type guard
• Using an in type guard
• Using a user-defined type guard with a type predicate
• Using a user-defined type guard with an assertion signature
• Understanding the discriminated union pattern
• Quiz