Route parameters in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when accessing a route with parameters?

Consider this NestJS controller method:

  @Get('user/:id')
  getUser(@Param('id') id: string) {
    return `User ID is ${id}`;
  }

What will be the response body when a client requests /user/42?

A "User ID is user/42"

B "User ID is :id"

C "User ID is 42"

D "User ID is undefined"

💡 Hint

Route parameters are extracted from the URL and passed to the method.

The @Param('id') decorator extracts the 'id' part from the URL path. So when the URL is /user/42, id equals '42'. The method returns the string with that value.

📝 Syntax

intermediate

Which option correctly defines a route with two parameters?

You want to create a GET route in NestJS that matches URLs like /order/123/item/456 and extracts both orderId and itemId as parameters.
Which method signature is correct?

A

@Get('order/:orderId/item/:itemId')
getOrderItem(@Param('orderId') orderId: string, itemId: string) { return {orderId, itemId}; }

B

@Get('order/:orderId/item/:itemId')
getOrderItem(@Param() params: {orderId: string, itemId: string}) { return params; }

C

@Get('order/:orderId/item/:itemId')
getOrderItem(@Param('orderId', 'itemId') ids: string[]) { return ids; }

D

@Get('order/:orderId/item/:itemId')
getOrderItem(@Param('orderId') orderId: string, @Param('itemId') itemId: string) { return {orderId, itemId}; }

💡 Hint

Each parameter must be decorated separately or use @Param() to get all.

Option D correctly uses @Param('orderId') and @Param('itemId') to extract each parameter individually. Option D is invalid because @Param() returns an object but the type annotation is incorrect. Option D uses invalid syntax for @Param. Option D misses @Param on itemId.

🔧 Debug

advanced

Why does this route parameter extraction fail?

Given this controller method:

@Get('product/:id')
getProduct(@Param() id: string) {
  return `Product ID: ${id}`;
}

What error or issue will occur when calling /product/10?

A TypeError because id is an object, not a string

B Returns 'Product ID: 10' correctly

C SyntaxError due to missing colon in route

D Runtime error because @Param() is missing argument

💡 Hint

Check what @Param() returns when no argument is given.

@Param() without an argument returns an object with all parameters. Assigning it to a string variable causes a type mismatch. Using it as a string causes a TypeError.

🧠 Conceptual

advanced

How does NestJS handle optional route parameters?

You want to define a route where the parameter category is optional, matching both /products and /products/electronics.
Which route path correctly supports this?

A @Get('products/(category)')

B @Get('products/:category?')

C @Get('products/:category')

D @Get('products/:category*')

💡 Hint

Optional parameters use a question mark in the path.

In NestJS (and Express), adding a question mark after a parameter makes it optional. So ':category?' matches zero or one segment. Other options are invalid syntax or mean different things.

❓ state_output

expert

What is the output of this controller with nested route parameters?

Given this NestJS controller snippet:

@Controller('users')
export class UserController {
  @Get(':userId/posts/:postId')
  getPost(@Param() params: { userId: string; postId: string }) {
    return `User ${params.userId} Post ${params.postId}`;
  }
}

What is the response body when a client requests /users/7/posts/15?

A "User 7 Post 15"

B "User :userId Post :postId"

C "User 7 Post :postId"

D "User undefined Post undefined"

💡 Hint

The @Param() decorator without argument returns an object with all parameters.

The params object contains userId and postId extracted from the URL. So the returned string uses those values correctly.

Practice

1. What does the : symbol represent in a NestJS route path like @Get('users/:id')?

easy

A. It is used to comment out the route path.

B. It defines a query parameter to be passed in the URL.

C. It indicates a static part of the URL path.

D. It marks id as a dynamic route parameter to capture from the URL.

Solution

1. Step 1: Understand route path syntax

   The colon : before id in @Get('users/:id') means id is a variable part of the URL.

2. Step 2: Identify the role of dynamic parameters

   This dynamic parameter allows the route to accept different id values from the URL, making the route flexible.

3. Final Answer:

   It marks id as a dynamic route parameter to capture from the URL. -> Option D

4. Quick Check:

   Colon means dynamic parameter = A [OK]

Hint: Colon before name means dynamic URL part [OK]

Common Mistakes:

• Thinking colon means query parameter
• Confusing static and dynamic parts
• Assuming colon is for comments

2. Which of the following is the correct way to access a route parameter named userId inside a NestJS controller method?

easy

A. @Body('userId') userId: string

B. @Query('userId') userId: string

C. @Param('userId') userId: string

D. @Headers('userId') userId: string

Solution

1. Step 1: Identify decorator for route parameters

   In NestJS, @Param() decorator is used to extract route parameters from the URL.

2. Step 2: Match parameter name

   Using @Param('userId') correctly accesses the userId parameter from the route.

3. Final Answer:

   @Param('userId') userId: string -> Option C

4. Quick Check:

   @Param for route params = C [OK]

Hint: Use @Param('name') to get route parameters [OK]

Common Mistakes:

• Using @Query instead of @Param for route params
• Trying to get route params from @Body or @Headers
• Forgetting to specify parameter name in @Param

3. Given this NestJS controller method:

@Get('products/:id')
getProduct(@Param('id') id: string) {
  return `Product ID is ${id}`;
}

What will be the response when a client requests /products/42?

medium

A. "Product ID is 42"

B. "Product ID is id"

C. "Product ID is undefined"

D. 404 Not Found error

Solution

1. Step 1: Understand route parameter extraction

   The @Param('id') decorator extracts the value 42 from the URL /products/42.

2. Step 2: Check returned string

   The method returns the string with the extracted id interpolated: "Product ID is 42".

3. Final Answer:

   "Product ID is 42" -> Option A

4. Quick Check:

   Route param 'id' = 42 in output [OK]

Hint: Route param value replaces :id in URL [OK]

Common Mistakes:

• Expecting literal 'id' instead of value
• Not using @Param to get the value
• Assuming undefined if param missing

4. What is wrong with this NestJS controller method?

@Get('orders/:orderId')
getOrder(@Param() orderId: string) {
  return `Order: ${orderId}`;
}

medium

A. Missing parameter name inside @Param() decorator.

B. Route path should not have colon before orderId.

C. Method should use @Query instead of @Param.

D. Return statement syntax is invalid.

Solution

1. Step 1: Check @Param usage

   The @Param() decorator without an argument returns an object of all params, not a single string.

2. Step 2: Identify mismatch in parameter type

   The method expects orderId as a string, but @Param() without argument returns an object, causing a type mismatch.

3. Final Answer:

   Missing parameter name inside @Param() decorator. -> Option A

4. Quick Check:

   @Param needs name for single param = B [OK]

Hint: Use @Param('name') for single param, else get object [OK]

Common Mistakes:

• Omitting parameter name in @Param() when expecting string
• Confusing @Param with @Query
• Thinking colon is wrong in route path

5. You want to create a NestJS route that accepts two parameters: category and itemId. Which of the following route paths and method signatures correctly captures both parameters?

hard

A. @Get('shop/:category/:itemId')getItem(@Body('category') category: string, @Body('itemId') itemId: string)

B. @Get('shop/:category/:itemId')getItem(@Param('category') category: string, @Param('itemId') itemId: string)

C. @Get('shop/:category/:itemId')getItem(@Query('category') category: string, @Query('itemId') itemId: string)

D. @Get('shop/:category/:itemId')getItem(@Param() params: string)

Solution

1. Step 1: Verify route path with two parameters

   The path 'shop/:category/:itemId' correctly defines two dynamic route parameters.

2. Step 2: Check method parameters for correct decorators

   Using @Param('category') and @Param('itemId') extracts each parameter individually as strings.

3. Final Answer:

   @Get('shop/:category/:itemId') with getItem(@Param('category') category: string, @Param('itemId') itemId: string) -> Option B

4. Quick Check:

   Use @Param('name') for each route param = D [OK]

Hint: Use @Param('name') for each route parameter [OK]

Common Mistakes:

• Using @Param() with string instead of object
• Using @Query or @Body instead of @Param for route params
• Not matching parameter names exactly