Custom type guard functions in Typescript - Mini Project: Build & Apply

Custom type guard functions

📖 Scenario: You are working on a TypeScript project where you receive data that can be either a string or a number. You want to safely check the type of the data before using it.

🎯 Goal: Build custom type guard functions to check if a value is a string or a number, then use these functions to safely handle the data.

📋 What You'll Learn

Create a variable with mixed type data

Create two custom type guard functions: isString and isNumber

Use these type guard functions in an if statement to check the data type

Print the type and value of the data using the type guards

💡 Why This Matters

🌍 Real World

Custom type guards help you safely work with data that can have multiple types, such as user input or API responses.

💼 Career

Knowing how to write type guards is important for TypeScript developers to avoid runtime errors and improve code safety.

1

Create a variable with mixed type data

Create a variable called data and assign it the value "hello" with the type string | number.

Typescript

// Create a variable called data with type string | number and value "hello"
// Your code here

Hint

Use let data: string | number = "hello"; to declare the variable.

2

Create custom type guard functions

Create two functions: isString and isNumber. Each should take a parameter value of type unknown and return a type predicate: value is string for isString and value is number for isNumber. Use typeof inside each function to check the type.

Typescript

let data: string | number = "hello";
// Create function isString(value: unknown): value is string
// Create function isNumber(value: unknown): value is number
// Your code here

Hint

Use typeof value === "string" and typeof value === "number" inside the functions.

3

Use type guard functions to check data type

Write an if statement that uses isString(data) to check if data is a string, and an else if statement that uses isNumber(data) to check if data is a number.

Typescript

let data: string | number = "hello";

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

function isNumber(value: unknown): value is number {
    return typeof value === "number";
}

// Use if statement with isString(data) and else if with isNumber(data)
// Your code here

Hint

Use if (isString(data)) { ... } and else if (isNumber(data)) { ... }.

4

Print the type and value using type guards

Inside the if block for isString(data), write console.log(`Data is a string: ${data}`). Inside the else if block for isNumber(data), write console.log(`Data is a number: ${data}`).

Typescript

let data: string | number = "hello";

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

function isNumber(value: unknown): value is number {
    return typeof value === "number";
}

if (isString(data)) {
    // Print data as string
    // Your code here
} else if (isNumber(data)) {
    // Print data as number
    // Your code here
}

Hint

Use console.log(`Data is a string: ${data}`) and console.log(`Data is a number: ${data}`).

Practice

1. What is the main purpose of a custom type guard function in TypeScript?

easy

A. To declare a new type alias

B. To convert a value from one type to another

C. To check if a value matches a specific type and help TypeScript narrow types

D. To throw an error when a type mismatch occurs

Solution

1. Step 1: Understand what a type guard does

   A type guard checks if a value is of a certain type at runtime.

2. Step 2: Recognize the benefit in TypeScript

   It helps TypeScript narrow the type, making code safer and clearer.

3. Final Answer:

   To check if a value matches a specific type and help TypeScript narrow types -> Option C

4. Quick Check:

   Type guard = check type + narrow [OK]

Hint: Type guards confirm type and help TypeScript narrow it [OK]

Common Mistakes:

• Thinking type guards convert types
• Confusing type guards with error handling
• Believing type guards declare new types

2. Which of the following is the correct syntax for a custom type guard function named isString that checks if a value is a string?

easy

A. function isString(value: any): value is string { return typeof value === 'string'; }

B. function isString(value: any): boolean { return typeof value === 'string'; }

C. function isString(value: any): string { return typeof value === 'string'; }

D. function isString(value: any): value is boolean { return typeof value === 'string'; }

Solution

1. Step 1: Identify the return type for a type guard

   A custom type guard returns a type predicate like value is string.

2. Step 2: Check the function body

   The function should return true if typeof value === 'string'.

3. Final Answer:

   function isString(value: any): value is string { return typeof value === 'string'; } -> Option A

4. Quick Check:

   Type guard syntax = value is Type [OK]

Hint: Type guard return type uses 'value is Type' syntax [OK]

Common Mistakes:

• Using boolean instead of type predicate as return type
• Returning wrong type predicate
• Returning the type instead of a boolean

3. What will be the output of the following TypeScript code?

function isNumber(value: any): value is number {
  return typeof value === 'number';
}

const input: any = 42;
if (isNumber(input)) {
  console.log(input.toFixed(2));
} else {
  console.log('Not a number');
}

medium

A. 42.00

B. Not a number

C. Error: toFixed is not a function

D. undefined

Solution

1. Step 1: Check the type guard function behavior

   The function isNumber returns true if value is a number.

2. Step 2: Analyze the if condition and output

   Since input is 42 (a number), the if block runs and input.toFixed(2) outputs '42.00'.

3. Final Answer:

   42.00 -> Option A

4. Quick Check:

   isNumber(42) = true, so prints 42.00 [OK]

Hint: If type guard true, use narrowed type safely [OK]

Common Mistakes:

• Assuming input is not a number
• Expecting runtime error on toFixed
• Confusing output with string conversion

4. Identify the error in this custom type guard function and choose the fix:

function isArray(value: any): value is Array {
  return Array.isArray(value);
}

medium

A. Change function name to 'isArrayType'

B. Change return type to 'value is any[]' instead of 'value is Array'

C. Add explicit type annotation to parameter 'value: unknown'

D. Replace 'Array.isArray' with 'typeof value === "array"'

Solution

1. Step 1: Understand the return type syntax

   The return type should be a type predicate with a specific type, e.g., value is any[] for arrays.

2. Step 2: Identify the incorrect type usage

   Array alone is a constructor, not a type for arrays. Use any[] or a typed array instead.

3. Final Answer:

   Change return type to 'value is any[]' instead of 'value is Array' -> Option B

4. Quick Check:

   Array type predicate = value is any[] [OK]

Hint: Use 'value is Type[]' for arrays in type guards [OK]

Common Mistakes:

• Using 'Array' instead of 'any[]' or typed array
• Changing function name unnecessarily
• Using typeof 'array' which is invalid

5. Given the following types and function, which implementation correctly narrows the type to Dog using a custom type guard?

interface Dog { bark(): void; }
interface Cat { meow(): void; }

type Pet = Dog | Cat;

function isDog(pet: Pet): pet is Dog {
  // Which implementation is correct?
}

hard

A. return pet.meow === undefined;

B. return pet instanceof Dog;

C. return typeof pet.bark === 'function';

D. return (pet as Dog).bark !== undefined;

Solution

1. Step 1: Understand the interfaces and type guard goal

   We want to check if pet has a bark method to confirm it's a Dog.

2. Step 2: Evaluate the implementations

   Directly accessing pet.bark or pet.meow causes TypeScript compile errors because those properties do not exist on the union type Pet.
   pet instanceof Dog does not work because Dog is an interface, not a class with a constructor.
   Using (pet as Dog).bark !== undefined compiles due to the type assertion and performs a runtime check to see if the bark property exists.

3. Final Answer:

   return (pet as Dog).bark !== undefined; -> Option D

4. Quick Check:

   Type assertion enables property check in union type guards [OK]

Hint: Use type assertion inside type guards for union property checks [OK]

Common Mistakes:

• Using instanceof with interfaces
• Directly accessing properties on union types
• Checking absence of other type's method