Git turns a plain folder into the repository from yesterday. You run git init in a folder, and from then on Git can track it. You make changes, tell Git which ones to record, and it saves a snapshot with your name, the time, and a message. That snapshot is a commit, tomorrow's word. Git's history is a chain of those snapshots. You can compare any two, roll back to any one, or start a separate line of changes and merge it in later. Days 21 to 24 cover those pieces one at a time. Git is local by default. Nothing leaves your machine until you push to a remote like GitHub. That's why you can work on a plane and sync later. Quick check: Git and GitHub. Which one is the tool on your computer, and which one is the website? ## SPOKEN LINES - Two of you edit the same file on the same afternoon. Whoever saves last wins, and the other person's hour of work is gone. - - GIT - - Git is a version control system. It watches your project folder, and when you tell it to, it records a snapshot with a note about what changed. - - You can undo a change from six months ago, two people can edit the same project at once, and a risky idea can live in its own line and get thrown away. - - So Git is the system that tracks every change to your code. Tomorrow: Commit, the snapshot Git takes when you tell it to. ## NEXT Day 21: Commit

Yesterday's package needs somewhere to be developed. That's the repository. Think of it as a project folder with a memory. Every change is recorded. Nothing is lost when you overwrite a file, because the earlier version is still in the history. A repo is also the unit of sharing. Open source packages on npm and PyPI almost always point back to a public repo, usually on GitHub, where you can read the source and report bugs. Two copies is normal. The local repo on your machine, the remote repo on GitHub or GitLab. You work locally and push changes to the remote when they're ready. Tomorrow covers the tool that makes that work. Quick check: You delete a file from a repo by accident. Is it gone for good? ## SPOKEN LINES - Your first Monday on a new team, and nobody sends you the code. The only instruction is clone the repo. One folder shows up on your laptop, holding the whole app plus three years of changes. - - REPOSITORY - - A repository is the home folder of a project. It keeps the full history: every version, who changed what, and when. - - You can open a file the way it looked six months ago, see who wrote line 40 and why, and get everything back if your laptop dies. - - So a repository is where a project's code and its history live. Tomorrow: Git, the tool that writes that history. ## NEXT Day 20: Git

Three words from the last three days, and they overlap on purpose: A library is the code. A package is the code wrapped for distribution, with a name and a version. A dependency is a package your project has installed and relies on. A package has a small file that says what it's called, what version it is, and which other packages it needs. That's how the dependency tree from yesterday gets built: the package manager reads that file, fetches what's listed, then reads those files, and keeps going. Your own project is a package too. package.json at the top of a JavaScript project describes it the same way. Quick check: You run pip install requests. Which of the three words above describes what pip just downloaded? ## SPOKEN LINES - A client wants card payments by Friday and you have never touched payment code. You type npm install stripe, and seconds later your project can take a card. - - PACKAGE - - A package is code bundled up with a name, a version number and a list of what it needs, so a tool can download and install it. npm does that for JavaScript, pip does it for Python. - - Stripe handles the card payment, express handles the routes, and dayjs turns a timestamp into 3 minutes ago. - - So a package is code boxed up so it can be installed and reused. Tomorrow: Repository, the folder where your whole project and its history live. ## NEXT Day 19: Repository

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