Custom type guard functions in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a custom type guard function that checks if a value is a string.

Typescript

function isString(value: unknown): value is string {
  return typeof value === [1];
}

Drag options to blanks, or click blank then click option'

A"string"

B"number"

C"boolean"

D"object"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong string in the typeof comparison.

Forgetting to put quotes around the type name.

The typeof operator returns a string describing the type. To check if a value is a string, compare it to "string".

2fill in blank

medium

Complete the code to create a custom type guard that checks if an object has a 'name' property of type string.

Typescript

function hasName(obj: unknown): obj is { name: string } {
  return typeof obj === [1] &&
         obj !== null &&
         'name' in obj &&
         typeof (obj as any).name === 'string';
}

Drag options to blanks, or click blank then click option'

A"object"

B"number"

C"string"

D"boolean"

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for 'string' instead of 'object' in typeof.

Not checking for null before accessing properties.

To check if obj is an object, use typeof obj === "object". This ensures obj is not a primitive type.

3fill in blank

hard

Fix the error in the custom type guard function that checks if a value is an array of numbers.

Typescript

function isNumberArray(value: unknown): value is number[] {
  return Array.isArray(value) && value.every(item => typeof item === [1]);
}

Drag options to blanks, or click blank then click option'

A"string"

B"number"

C"object"

D"boolean"

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for 'string' instead of 'number'.

Not using Array.isArray to check if value is an array.

The typeof operator should check if each item is a "number" to confirm the array contains only numbers.

4fill in blank

hard

Fill both blanks to create a custom type guard that checks if an object is a Person with a 'name' string and 'age' number.

Typescript

function isPerson(obj: unknown): obj is { name: string; age: number } {
  return typeof obj === [1] &&
         obj !== null &&
         typeof (obj as any).name === 'string' &&
         typeof (obj as any).age === [2];
}

Drag options to blanks, or click blank then click option'

A"object"

B"string"

C"number"

D"boolean"

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong type strings in typeof checks.

Not checking for null before property access.

First, check that obj is an object. Then check that obj.name is a string and obj.age is a number.

5fill in blank

hard

Fill all three blanks to create a custom type guard that checks if a value is a tuple of [string, number, boolean].

Typescript

function isTuple(value: unknown): value is [string, number, boolean] {
  return Array.isArray(value) &&
         value.length === [1] &&
         typeof value[0] === [2] &&
         typeof value[1] === [3];
}

Drag options to blanks, or click blank then click option'

A3

B"string"

C"number"

D"boolean"

Attempts:

3 left

💡 Hint

Common Mistakes

Not checking the length of the array.

Using wrong type strings in typeof checks.

The tuple has length 3. The first element is a string, the second is a number, and the third is a boolean (not checked here but could be added).

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