A library becomes a dependency the moment your project relies on it. Dependencies chain. Your app depends on a charting library. The charting library depends on a math library. Your app now depends on both, even though you never chose the second one. That chain is called the dependency tree, and on a real project it's big. Two things follow from that. Your project's dependency list is part of the project. Anyone who clones it runs one install command and gets the same set. And you don't own that code. When a dependency ships a bug or a security fix, you need to update. Keeping dependencies current is a real, ongoing job. Quick check: You install a library and it pulls in six others you didn't ask for. Are those six dependencies of your project? ## SPOKEN LINES - You delete one tiny date library. Every timestamp in the app breaks at once, and the build fails on your laptop. The app was leaning on it, and on a few hundred more like it. - - DEPENDENCY - - A dependency is outside code your project needs in order to run. Each library you add brings its own dependencies, and those bring their own. - - Your date library is one, the tiny helper it needs is another, and the 300 you never picked came along for the ride. - - So a dependency is code from outside that your app relies on to run. Tomorrow: Package, how that code gets boxed up and installed. ## NEXT Day 18: Package

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