Part 2: Routing and Data¶

Verified status as of March 28, 2026. Runtime note: FastFN auto-installs function-local dependencies from requirements.txt / package.json; host runtimes are required in fastfn dev --native, while fastfn dev depends on a running Docker daemon.

Quick View¶

Complexity: Intermediate
Typical time: 25-35 minutes
Outcome: dynamic path/query/body handling with explicit validation errors

1. Path params and catch-all¶

Create dynamic route files under functions/.

Node.jsPythonRustPHP

File: functions/tasks/[id].js

exports.handler = async (_event, { id }) => ({ status: 200, body: { task_id: id } });

File: functions/reports/[...slug].js

exports.handler = async (_event, { slug }) => ({ status: 200, body: { path: slug } });

File: functions/tasks/[id].py

def handler(_event, params):
    return {"status": 200, "body": {"task_id": params.get("id")}}

File: functions/reports/[...slug].py

def handler(_event, params):
    return {"status": 200, "body": {"path": params.get("slug")}}

File: functions/tasks/[id].rs

use serde_json::{json, Value};

pub fn handler(_event: Value, params: Value) -> Value {
    json!({"status": 200, "body": {"task_id": params.get("id").and_then(|v| v.as_str()).unwrap_or("")}})
}

File: functions/tasks/[id].php

<?php
function handler(array $event, array $params): array {
    return ['status' => 200, 'body' => ['task_id' => $params['id'] ?? '']];
}

Validation curls (same for all runtimes):

curl -sS 'http://127.0.0.1:8080/tasks/42'
curl -sS 'http://127.0.0.1:8080/reports/2026/03/daily'

2. Query params with defaults¶

Node.jsPythonRustPHP

File: functions/tasks/search.js

exports.handler = async (event) => {
  const q = event.query?.q;
  const page = Number(event.query?.page || "1");
  if (!q) return { status: 400, body: { error: "q is required" } };
  return { status: 200, body: { q, page } };
};

File: functions/tasks/search.py

def handler(event):
    query = event.get("query") or {}
    q = query.get("q")
    page = int(query.get("page", "1"))
    if not q:
        return {"status": 400, "body": {"error": "q is required"}}
    return {"status": 200, "body": {"q": q, "page": page}}

File: functions/tasks/search.rs

use serde_json::{json, Value};

pub fn handler(event: Value) -> Value {
    let q = event.get("query").and_then(|x| x.get("q")).and_then(|x| x.as_str());
    if q.is_none() {
        return json!({"status": 400, "body": {"error": "q is required"}});
    }
    json!({"status": 200, "body": {"q": q.unwrap(), "page": 1}})
}

File: functions/tasks/search.php

<?php
function handler(array $event): array {
    $query = $event['query'] ?? [];
    $q = $query['q'] ?? null;
    $page = intval($query['page'] ?? '1');
    if (!$q) return ['status' => 400, 'body' => ['error' => 'q is required']];
    return ['status' => 200, 'body' => ['q' => $q, 'page' => $page]];
}

Runtime curls:

Node.jsPythonRustPHP

curl -sS 'http://127.0.0.1:8080/tasks/search?page=2'
curl -sS 'http://127.0.0.1:8080/tasks/search?q=fastfn'

curl -sS 'http://127.0.0.1:8080/tasks/search?page=2'
curl -sS 'http://127.0.0.1:8080/tasks/search?q=fastfn'

curl -sS 'http://127.0.0.1:8080/tasks/search?page=2'
curl -sS 'http://127.0.0.1:8080/tasks/search?q=fastfn'

curl -sS 'http://127.0.0.1:8080/tasks/search?page=2'
curl -sS 'http://127.0.0.1:8080/tasks/search?q=fastfn'

3. JSON body parsing and validation¶

Node.jsPythonRustPHP

File: functions/tasks/post.js

exports.handler = async (event) => {
  let payload;
  try { payload = JSON.parse(event.body || "{}"); }
  catch { return { status: 400, body: { error: "invalid JSON body" } }; }
  if (!payload.title || typeof payload.title !== "string") {
    return { status: 422, body: { error: "title must be a non-empty string" } };
  }
  return { status: 201, body: { id: 3, title: payload.title } };
};

File: functions/tasks/post.py

import json

def handler(event):
    try:
        payload = json.loads(event.get("body") or "{}")
    except Exception:
        return {"status": 400, "body": {"error": "invalid JSON body"}}
    if not payload.get("title"):
        return {"status": 422, "body": {"error": "title must be a non-empty string"}}
    return {"status": 201, "body": {"id": 3, "title": payload["title"]}}

File: functions/tasks/post.rs

use serde_json::{json, Value};

pub fn handler(event: Value) -> Value {
    let payload = event.get("body").and_then(|b| b.as_str()).unwrap_or("{}");
    let parsed: Value = serde_json::from_str(payload).unwrap_or(json!({"_error": "invalid"}));
    if parsed.get("_error").is_some() {
        return json!({"status": 400, "body": {"error": "invalid JSON body"}});
    }
    if parsed.get("title").and_then(|x| x.as_str()).unwrap_or("").is_empty() {
        return json!({"status": 422, "body": {"error": "title must be a non-empty string"}});
    }
    json!({"status": 201, "body": {"id": 3, "title": parsed["title"]}})
}

File: functions/tasks/post.php

<?php
function handler(array $event): array {
    $raw = $event['body'] ?? '{}';
    $payload = json_decode($raw, true);
    if (!is_array($payload)) return ['status' => 400, 'body' => ['error' => 'invalid JSON body']];
    if (empty($payload['title']) || !is_string($payload['title'])) {
        return ['status' => 422, 'body' => ['error' => 'title must be a non-empty string']];
    }
    return ['status' => 201, 'body' => ['id' => 3, 'title' => $payload['title']]];
}

Runtime curls:

Node.jsPythonRustPHP

curl -sS -X POST 'http://127.0.0.1:8080/tasks' -H 'Content-Type: application/json' -d '{bad'
curl -sS -X POST 'http://127.0.0.1:8080/tasks' -H 'Content-Type: application/json' -d '{}'
curl -sS -X POST 'http://127.0.0.1:8080/tasks' -H 'Content-Type: application/json' -d '{"title":"Write docs"}'

curl -sS -X POST 'http://127.0.0.1:8080/tasks' -H 'Content-Type: application/json' -d '{bad'
curl -sS -X POST 'http://127.0.0.1:8080/tasks' -H 'Content-Type: application/json' -d '{}'
curl -sS -X POST 'http://127.0.0.1:8080/tasks' -H 'Content-Type: application/json' -d '{"title":"Write docs"}'

curl -sS -X POST 'http://127.0.0.1:8080/tasks' -H 'Content-Type: application/json' -d '{bad'
curl -sS -X POST 'http://127.0.0.1:8080/tasks' -H 'Content-Type: application/json' -d '{}'
curl -sS -X POST 'http://127.0.0.1:8080/tasks' -H 'Content-Type: application/json' -d '{"title":"Write docs"}'

curl -sS -X POST 'http://127.0.0.1:8080/tasks' -H 'Content-Type: application/json' -d '{bad'
curl -sS -X POST 'http://127.0.0.1:8080/tasks' -H 'Content-Type: application/json' -d '{}'
curl -sS -X POST 'http://127.0.0.1:8080/tasks' -H 'Content-Type: application/json' -d '{"title":"Write docs"}'

Flow diagram¶

flowchart LR
  A["Route path"] --> B["Path params"]
  B --> C["Query parsing"]
  C --> D["Body parsing"]
  D --> E["Validation result"]
  E --> F["HTTP response"]

Troubleshooting¶

wrong handler not invoked: verify filename prefixes and folder names
params missing: verify [id] or [...slug] pattern
body parse errors: confirm Content-Type: application/json and valid JSON syntax

Next step¶

Go to Part 3: Configuration and Secrets

Last reviewed: March 28, 2026 · Docs on fastfn.dev

Part 2: Routing and Data¶

Quick View¶

1. Path params and catch-all¶

2. Query params with defaults¶

3. JSON body parsing and validation¶

Flow diagram¶

Troubleshooting¶

Next step¶

Related links¶