Yesterday's framework and today's library both mean "code you didn't write". The line between them is control. A framework calls you. It owns the app's structure and runs your code at the points it defines. A library waits to be called. Your code owns the structure and uses the library for one job. React is often called a framework, but strictly speaking it's a library: it handles rendering the screen and you decide everything else. Next.js wraps React into a full framework. People blur these terms all the time, so don't stress when you hear them swapped. A typical project uses one framework and dozens of libraries. Quick check: You add a piece of code that draws charts, and you call it from three different screens. Library or framework? ## SPOKEN LINES - Under every message your app says 3 minutes ago, and it is wrong for anyone in a different time zone. You spent a whole weekend on a feature somebody else finished years ago. - - LIBRARY - - A library is a collection of code someone else wrote, packaged so you can pull it into your project and call it. You stay in charge. Your code calls the library when it needs it and ignores it the rest of the time. - - A date library gives you 3 minutes ago, a chart library draws the bar graph on your dashboard, and an email library sends the receipt. - - So a library is ready-made code your app calls when it needs it. Tomorrow: Dependency, what happens once your app cannot run without that library. ## NEXT Day 17: Dependency

Take everything from the last week. Routes, requests, responses, status codes, JSON, CRUD. A framework ships with all of that wired up. You define a route, write the handler, and the framework takes care of receiving the HTTP request, running your code, and sending the response back with the right status code. A backend framework (Express, Django, Rails) handles the server side. A frontend framework (React, Vue, Angular) handles screens and what happens when the user taps things. The trade: you write less, and you write it the framework's way. Folder layout, naming, how data flows. Fighting those conventions is a bad time. Quick check: Name one framework you've heard of, even if you're not sure what it does. ## SPOKEN LINES - Three months late, and your food delivery app still cannot take an order. You wrote your own request handling, your own routing, your own database layer, and every piece has a bug. Nobody who ships on time writes all of that from scratch. - - FRAMEWORK - - A framework is a pre-built structure you build your app inside. It handles the plumbing every app needs and gives you set places to put your own code. The framework is in charge, and it calls your code when it decides to. - - Rails and Django run the server side, React builds the screens, and Express handles routes in JavaScript. - - So a framework is a ready-made structure with rules for where your code goes. Tomorrow: Library, code you call instead of code that calls you. ## NEXT Day 16: Library

Day 7 gave us the endpoint: the address a request goes to. Today is what happens on the other side of that address. A route pairs a path with a handler. The path is the pattern, the handler is the code. GET /tasks runs the code that lists tasks. POST /tasks runs the code that creates one. GET /tasks/12 runs the code that loads task number 12. Same path, different method, different handler. That's why the CRUD operations from yesterday can share one endpoint. Routes can hold variables. /tasks/12 and /tasks/13 both match the pattern /tasks/:id. The server pulls out the id and passes it to the handler. Quick check: A request comes in as DELETE /tasks/12. In plain words, what should the matching handler do? ## SPOKEN LINES - You tap a friend's profile and land on the settings screen instead. The address bar says users, 4521, and the server sent back the wrong page. One rule on the server pointed that path at the wrong code. - - ROUTE - - A route is a rule on the server that matches a path to the code that handles it. The server keeps a list of them. A request comes in, the path is checked against the list, and the matching code runs. - - The cart path runs the cart code, users 4521 loads one profile, and orders 88 pulls up a single order. - - So a route is the path that sends a request to the right code. Tomorrow: Framework, the ready-made structure most apps are built inside. ## NEXT Day 15: Framework