Dynamic Responses
Learn how to create dynamic responses that change based on request data using moclojer's powerful template system. Make your mocks more realistic and flexible.
In the previous tutorial, you created static endpoints that always return the same response. Now you'll learn how to make your responses dynamic - changing based on the data sent in the request. This makes your mocks much more realistic and useful for testing.
What you'll learn
How to use path parameters in URLs
How to capture and use query parameters
How to access JSON data from request bodies
How to use template variables to create dynamic content
How to test dynamic responses
What you'll build
An enhanced user API that responds dynamically to different inputs:
GET /users/:id- Returns user data based on the ID in the URLGET /users- Filters users based on query parametersPOST /users- Creates users with data from the request bodyGET /search- Search functionality with dynamic results
Prerequisites
Completed the Your First Mock Server tutorial
moclojer running on your system
Basic understanding of HTTP requests
Step 1: Understanding template variables
Template variables are placeholders in your responses that get replaced with actual data from the request. They use the {{variable}} syntax.
Available template variables:
{{path-params.name}}- Values from URL path (e.g.,:id,:username){{query-params.name}}- Values from query string (e.g.,?search=value){{json-params.name}}- Values from JSON request body{{header-params.name}}- Values from HTTP headers
Step 2: Path parameters
Path parameters let you capture parts of the URL and use them in your response.
Create a new moclojer.yml file:
- endpoint:
method: GET
path: /users/:id
response:
status: 200
headers:
Content-Type: application/json
body: >
{
"id": "{{path-params.id}}",
"name": "User {{path-params.id}}",
"email": "user{{path-params.id}}@example.com",
"created_at": "2024-01-15T10:30:00Z"
}
- endpoint:
method: GET
path: /users/:id/profile/:section
response:
status: 200
headers:
Content-Type: application/json
body: >
{
"user_id": "{{path-params.id}}",
"section": "{{path-params.section}}",
"data": {
"title": "{{path-params.section}} for User {{path-params.id}}",
"last_updated": "2024-01-15T10:30:00Z"
}
}Test it:
Start your server and try these requests:
# Different user IDs
curl http://localhost:8000/users/123
curl http://localhost:8000/users/456
# Different profile sections
curl http://localhost:8000/users/123/profile/settings
curl http://localhost:8000/users/456/profile/preferencesNotice how the responses change based on the values in the URL!
Step 3: Query parameters
Query parameters come after the ? in URLs and let you pass additional data.
Add this endpoint to your configuration:
- endpoint:
method: GET
path: /users
response:
status: 200
headers:
Content-Type: application/json
body: >
{
"filters": {
"role": "{{query-params.role}}",
"department": "{{query-params.department}}",
"active": "{{query-params.active}}"
},
"pagination": {
"page": "{{query-params.page}}",
"limit": "{{query-params.limit}}"
},
"users": [
{
"id": 1,
"name": "Alice Johnson",
"role": "{{query-params.role}}",
"department": "{{query-params.department}}"
},
{
"id": 2,
"name": "Bob Smith",
"role": "{{query-params.role}}",
"department": "{{query-params.department}}"
}
]
}
- endpoint:
method: GET
path: /search
response:
status: 200
headers:
Content-Type: application/json
body: >
{
"query": "{{query-params.q}}",
"category": "{{query-params.category}}",
"results": [
{
"id": 1,
"title": "Result 1 for '{{query-params.q}}'",
"category": "{{query-params.category}}"
},
{
"id": 2,
"title": "Result 2 for '{{query-params.q}}'",
"category": "{{query-params.category}}"
}
],
"total": 2
}Test it:
# Filter users by role and department
curl "http://localhost:8000/users?role=admin&department=engineering"
# Search with different queries
curl "http://localhost:8000/search?q=javascript&category=tutorials"
curl "http://localhost:8000/search?q=python&category=examples"
# Pagination
curl "http://localhost:8000/users?page=2&limit=10"Step 4: JSON body parameters
For POST, PUT, and PATCH requests, you can access data from the JSON request body.
Add these endpoints:
- endpoint:
method: POST
path: /users
response:
status: 201
headers:
Content-Type: application/json
body: >
{
"id": 12345,
"name": "{{json-params.name}}",
"email": "{{json-params.email}}",
"role": "{{json-params.role}}",
"department": "{{json-params.department}}",
"created_at": "2024-01-15T10:30:00Z",
"message": "User '{{json-params.name}}' created successfully"
}
- endpoint:
method: PUT
path: /users/:id
response:
status: 200
headers:
Content-Type: application/json
body: >
{
"id": "{{path-params.id}}",
"name": "{{json-params.name}}",
"email": "{{json-params.email}}",
"role": "{{json-params.role}}",
"updated_at": "2024-01-15T10:30:00Z",
"message": "User {{path-params.id}} updated with name '{{json-params.name}}'"
}
- endpoint:
method: POST
path: /orders
response:
status: 201
headers:
Content-Type: application/json
body: >
{
"order_id": "ORD-{{json-params.user_id}}-12345",
"user_id": "{{json-params.user_id}}",
"items": [
{
"product": "{{json-params.product}}",
"quantity": "{{json-params.quantity}}",
"price": "{{json-params.price}}"
}
],
"total": "{{json-params.price}}",
"status": "confirmed",
"created_at": "2024-01-15T10:30:00Z"
}Test it:
# Create a new user
curl -X POST \
-H "Content-Type: application/json" \
-d '{
"name": "John Doe",
"email": "[email protected]",
"role": "developer",
"department": "engineering"
}' \
http://localhost:8000/users
# Update a user
curl -X PUT \
-H "Content-Type: application/json" \
-d '{
"name": "John Smith",
"email": "[email protected]",
"role": "senior developer"
}' \
http://localhost:8000/users/123
# Create an order
curl -X POST \
-H "Content-Type: application/json" \
-d '{
"user_id": "123",
"product": "Laptop",
"quantity": 1,
"price": "999.99"
}' \
http://localhost:8000/ordersStep 5: Header parameters
You can also access HTTP headers in your responses:
- endpoint:
method: GET
path: /profile
response:
status: 200
headers:
Content-Type: application/json
body: >
{
"user_agent": "{{header-params.User-Agent}}",
"authorization": "{{header-params.Authorization}}",
"content_type": "{{header-params.Content-Type}}",
"custom_header": "{{header-params.X-Custom-Header}}",
"message": "Headers received successfully"
}Test it:
curl -H "Authorization: Bearer token123" \
-H "X-Custom-Header: custom-value" \
http://localhost:8000/profileStep 6: Combining multiple parameters
Real APIs often use multiple types of parameters. Here's a comprehensive example:
- endpoint:
method: POST
path: /api/:version/users/:id/messages
response:
status: 201
headers:
Content-Type: application/json
body: >
{
"message_id": "MSG-{{path-params.id}}-{{json-params.recipient_id}}",
"api_version": "{{path-params.version}}",
"sender_id": "{{path-params.id}}",
"recipient_id": "{{json-params.recipient_id}}",
"subject": "{{json-params.subject}}",
"body": "{{json-params.message}}",
"priority": "{{query-params.priority}}",
"user_agent": "{{header-params.User-Agent}}",
"created_at": "2024-01-15T10:30:00Z",
"status": "sent"
}Test it:
curl -X POST \
-H "Content-Type: application/json" \
-H "User-Agent: MyApp/1.0" \
-d '{
"recipient_id": "456",
"subject": "Hello World",
"message": "This is a test message"
}' \
"http://localhost:8000/api/v1/users/123/messages?priority=high"Step 7: Handling missing parameters
moclojer gracefully handles missing parameters by showing them as empty strings:
- endpoint:
method: GET
path: /debug
response:
status: 200
headers:
Content-Type: application/json
body: >
{
"path_param": "{{path-params.missing}}",
"query_param": "{{query-params.missing}}",
"note": "Missing parameters appear as empty strings"
}Understanding data types
When using template variables, remember:
Strings need quotes:
"name": "{{json-params.name}}"Numbers don't need quotes:
"age": {{json-params.age}}Booleans don't need quotes:
"active": {{json-params.active}}
Example with mixed types:
- endpoint:
method: POST
path: /products
response:
status: 201
body: >
{
"name": "{{json-params.name}}",
"price": {{json-params.price}},
"in_stock": {{json-params.in_stock}},
"category": "{{json-params.category}}"
}Your complete dynamic configuration
Here's your complete moclojer.yml with all the dynamic examples:
# Path parameters
- endpoint:
method: GET
path: /users/:id
response:
status: 200
headers:
Content-Type: application/json
body: >
{
"id": "{{path-params.id}}",
"name": "User {{path-params.id}}",
"email": "user{{path-params.id}}@example.com",
"created_at": "2024-01-15T10:30:00Z"
}
# Query parameters
- endpoint:
method: GET
path: /search
response:
status: 200
headers:
Content-Type: application/json
body: >
{
"query": "{{query-params.q}}",
"category": "{{query-params.category}}",
"results": [
{
"id": 1,
"title": "Result 1 for '{{query-params.q}}'",
"category": "{{query-params.category}}"
}
]
}
# JSON body parameters
- endpoint:
method: POST
path: /users
response:
status: 201
headers:
Content-Type: application/json
body: >
{
"id": 12345,
"name": "{{json-params.name}}",
"email": "{{json-params.email}}",
"created_at": "2024-01-15T10:30:00Z",
"message": "User '{{json-params.name}}' created successfully"
}
# Combined parameters
- endpoint:
method: POST
path: /api/:version/users/:id/messages
response:
status: 201
headers:
Content-Type: application/json
body: >
{
"message_id": "MSG-{{path-params.id}}-{{json-params.recipient_id}}",
"api_version": "{{path-params.version}}",
"sender_id": "{{path-params.id}}",
"recipient_id": "{{json-params.recipient_id}}",
"subject": "{{json-params.subject}}",
"priority": "{{query-params.priority}}",
"created_at": "2024-01-15T10:30:00Z"
}
# Header parameters
- endpoint:
method: GET
path: /profile
response:
status: 200
headers:
Content-Type: application/json
body: >
{
"user_agent": "{{header-params.User-Agent}}",
"authorization": "{{header-params.Authorization}}",
"message": "Headers received successfully"
}What you've accomplished
✅ Path parameters - Capture values from URLs ✅ Query parameters - Use URL query strings ✅ JSON body parameters - Access request body data ✅ Header parameters - Read HTTP headers ✅ Combined parameters - Use multiple parameter types together ✅ Data type handling - Work with strings, numbers, and booleans
Next steps
Your mock APIs are now much more realistic and flexible! In the next tutorial, you'll learn how to organize multiple endpoints and create a complete API with proper structure.
👉 Multiple Endpoints Tutorial - Learn to organize and structure larger APIs
Need help?
Confused about templates? See Template System Overview
Want more examples? Check Common Patterns
Have questions? Join the community discussions
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