Route handlers for webhooks in NextJS - Step-by-Step Execution

Concept Flow - Route handlers for webhooks

Incoming HTTP Request

↓

Next.js Route Handler

↓

Parse Request Body

↓

Validate Webhook Signature

↓

Process Event

↓

Send 200 OK

↓

End

The webhook route handler receives a request, parses and validates it, then processes or rejects it accordingly.

Execution Sample

NextJS

export async function POST(req) {
  const body = await req.json();
  if (!isValidSignature(req)) {
    return new Response('Invalid signature', { status: 400 });
  }
  await handleEvent(body);
  return new Response('OK', { status: 200 });
}

This code handles a POST webhook request by validating the signature, processing the event, and responding.

Execution Table

Step	Action	Input/Condition	Result	Response Sent
1	Receive POST request	Request with JSON body and headers	Request object ready	No
2	Parse JSON body	Request body JSON	Parsed body object	No
3	Validate signature	Check signature header	Signature valid?	No
4	Signature valid?	Yes	Proceed to handle event	No
5	Handle event	Parsed body	Event processed	No
6	Send response	Event processed	Response 200 OK sent	Yes
7	If signature invalid	No	Send 400 error response	Yes

💡 Execution stops after sending either 200 OK or 400 error response.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5	Final
req	Incoming request object	Same	Same	Same	Same
body	undefined	Parsed JSON object	Parsed JSON object	Parsed JSON object	Parsed JSON object
signatureValid	undefined	undefined	true or false	true or false	true or false
response	undefined	undefined	undefined	undefined	Response object (200 or 400)

Key Moments - 3 Insights

Why do we check the signature before processing the event?

Checking the signature first (see step 3 and 4 in execution_table) ensures we only process requests from trusted sources, preventing fake or malicious webhook calls.

What happens if the signature is invalid?

If the signature is invalid (step 7), the handler immediately sends a 400 error response and stops processing, protecting the app from bad data.

Why do we parse the JSON body before validating the signature?

Parsing the JSON body (step 2) is needed to access the event data, but signature validation (step 3) usually uses headers and raw body; in Next.js, you may need to handle raw body carefully to validate signature correctly.

Visual Quiz - 3 Questions

Look at the execution_table, what response is sent if the signature is valid?

A 400 Error

B 200 OK

C No response sent

D 500 Server Error

💡 Hint

Check step 6 in the execution_table where response 200 OK is sent after valid signature.

At which step does the handler decide to reject the request due to invalid signature?

A Step 7

B Step 4

C Step 2

D Step 5

💡 Hint

Look at step 7 in the execution_table where 400 error is sent if signature is invalid.

If the JSON body parsing fails, what would happen in this flow?

A The handler sends 400 error due to invalid signature

B The handler sends 200 OK anyway

C The handler never reaches signature validation

D The handler processes event with empty body

💡 Hint

Parsing JSON happens at step 2; if it fails, the flow stops before signature validation at step 3.

Concept Snapshot

Next.js webhook route handlers receive POST requests.
Parse JSON body from request.
Validate webhook signature to ensure trust.
If valid, process event and respond 200 OK.
If invalid, respond 400 error and stop.
Always secure webhook endpoints to prevent fake calls.

Full Transcript

This visual execution trace shows how Next.js route handlers manage webhook POST requests. First, the handler receives the request and parses its JSON body. Then it validates the webhook signature to confirm the request is from a trusted source. If the signature is valid, the event is processed and a 200 OK response is sent. If the signature is invalid, the handler immediately sends a 400 error response and stops further processing. Variables like the request object, parsed body, signature validation status, and response evolve through these steps. Key beginner questions include why signature validation is done before processing, what happens on invalid signatures, and the importance of parsing the body correctly. The quiz tests understanding of response codes, decision points, and error handling in the flow.