What are type guards and how do you use them in TypeScript?

Q: What are type guards and how do you use them in TypeScript?

Learn the answer to "What are type guards and how do you use them in TypeScript?" with detailed explanations, code examples, and best practices on DeployU.

The Scenario

You are a frontend engineer at a social media company. You are writing a new function that takes a value of an unknown type and needs to perform a different operation depending on the type of the value.

You could use a series of if statements with typeof checks, but this would not be very elegant or type-safe.

The Challenge

Explain what type guards are in TypeScript and how you would use them to solve this problem. What are the key benefits of using type guards?

Wrong Approach

A junior engineer might not be aware of type guards. They might try to solve this problem by using a series of `if` statements with `typeof` checks, which would be a verbose and error-prone solution.

Addresses symptoms, not root cause

Right Approach

A senior engineer would know that type guards are the perfect tool for this job. They would be able to explain what type guards are and how to use them to create a function with a dynamic return type.

Step 1: Understand What Type Guards Are

A type guard is a function that returns a boolean value and that has a type predicate as its return type. A type predicate is a special return type that tells the TypeScript compiler that a value has a specific type if the function returns true.

Step 2: Write a Simple Type Guard

Here’s how we can write a simple type guard to check if a value is a string:

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

Step 3: Use the Type Guard

We can use the type guard in an if statement to narrow the type of a value:

function myFunc(myVar: unknown) {
  if (isString(myVar)) {
    // myVar is now of type string
    console.log(myVar.toUpperCase());
  }
}

The Benefits of Using Type Guards

Benefit	Description
Type Safety	Type guards provide compile-time type safety, which can help you to catch errors early.
Readability	Type guards make your code more readable by making it clear what type of data you are working with.
Reusability	You can reuse the same type guard in multiple places in your code.

Systematic, production-ready debugging

Practice Question

You want to create a type guard that checks if a value is a `number`. Which of the following would you use?

Questions