What is a DART File?Source Code

📖 What is a DART File?

A DART file is a source code file written in Dart, a programming language made by Google. In plain terms, it is a text file holding instructions a developer has written, the building blocks of an app or program, saved with the .dart extension.

Dart is the language most people meet through Flutter, Google's popular toolkit for building apps that run on phones, the web and desktop from one shared codebase. When a developer builds a Flutter app, the logic and the on-screen layout are written in Dart and stored across a set of .dart files. Each file is ordinary text, so it contains no images or compiled program, just the readable code that describes what the app should do and how it should look. A single small app might have a handful of these files, while a large one can have hundreds, each handling a different screen or piece of functionality.

A DART file holds source code instead of a ready-to-run program, so it is a stage in making software rather than the software itself. A developer writes and edits the .dart files, then a set of tools reads that code and turns it into something a device can actually run. That is the key thing to hold onto, a DART file is human-written instructions waiting to be built, which is why you open it to read or edit it, not to launch it like an app.

⚡ Quick Facts

🧱 What is Inside a DART File?

Opening a DART file reveals plain, fairly readable code, and even without knowing Dart you can usually make out its shape, which helps demystify what these files are.

Inside you will find things like functions, which are named blocks of instructions, and classes, which are blueprints for the objects an app works with. Dart uses a style of syntax similar to languages such as C, Java and JavaScript, with curly braces marking out blocks and semicolons ending statements, so it looks familiar to many programmers. A special function called main is the starting point, the very first code that runs when the program begins, which is why the file holding it is typically named main.dart. Here is a tiny example of what a DART file can look like inside.

// main.dart, the starting point of the program
void main() {
  print('Hello, Dart!');
  var total = add(3, 4);
  print('The total is $total');
}

int add(int a, int b) {
  return a + b;
}

That small file defines a starting point that prints a greeting, then a function that adds two numbers and reports the result. Real DART files grow far larger and more involved, but they are built from these same pieces, functions and classes written as readable text.

📱 Dart and Flutter

Most people who come across a DART file do so because of Flutter, so it is worth being clear on how the two relate, since they are often confused for one another.

Dart is the language, and Flutter is the framework built on top of it. Flutter is Google's toolkit for creating apps that run on iOS, Android, the web and desktop from a single set of source files, and the code you write for a Flutter app is written in Dart. In a Flutter project, the .dart files live mostly in a folder named lib, and the app starts from a file called main.dart. The screens and buttons you see in a Flutter app are described in Dart as widgets, small reusable pieces of interface, so a lot of the .dart code in a Flutter project is really a description of what the app should look like and how it should behave. The short version is that Flutter is what you are building and Dart is the language you build it in, which is why learning Flutter means writing a great many DART files.

⚙️ How Dart Code Runs

One of the more interesting things about Dart is how flexibly its code can be turned into a running program, and this flexibility is a big part of why Google built it.

Because a DART file is source code, it has to be processed before a device can run it, and Dart offers several routes. For mobile and desktop apps its compiler can turn the code ahead of time into native machine code, the fast low-level instructions a processor runs directly, which gives Flutter apps their smooth performance. For the web, Dart is compiled into JavaScript so it runs in any browser, since browsers do not understand Dart directly. During development there is a quicker path too, where the code runs on the Dart virtual machine using just-in-time compilation, and this is what powers Flutter's much-loved hot reload, the feature that lets a developer change the code and see the result in the running app almost instantly. All of these routes are handled by the Dart SDK, the software development kit that ships with the language and includes the tools and the Dart VM. As a developer you rarely think about which route is used, you simply run or build your project and the SDK does the rest.

📂 How to Open a DART File

Because a DART file is plain text, opening one to look at it is easy, and which tool you pick comes down to whether you just want to read the code or actually work on it.

Just to view what is inside, reach for any plain text editor, since the file is nothing but readable characters. Something as simple as Windows Notepad or the Mac TextEdit app displays the raw code, which does the job for a quick glance. For real work, though, developers use a code editor that understands Dart, most commonly Visual Studio Code with its Dart and Flutter extensions, or Android Studio and IntelliJ IDEA with the Dart plugin. These tools colour the code to make it readable, flag mistakes as you type and connect to the Dart tools so you can run and build your project. What you cannot do is double-click a DART file and have it launch like a program, because it is source code and needs to be built first. If you have been sent a .dart file and only want to read it, a text editor is the quickest way in, while anyone planning to edit or run it will want a proper Dart-aware editor.

🛠️ Working With DART Files

Reading a DART file is one thing, but actually writing and running Dart code takes a little setup, and the good news is that the tooling is well organised and free.

The foundation is the Dart SDK, which you install to get the language, its command-line tool and the Dart VM. If your goal is Flutter apps, you instead install the Flutter SDK, which bundles Dart along with everything Flutter needs. With that in place, you open your project in a Dart-aware editor and work across its .dart files, and the command-line tool lets you run a program with a simple run command or turn it into a finished build with a compile command. A DART file rarely stands alone, since a real project is a whole folder of them that import and use one another, together with a configuration file that lists the outside packages the project depends on. When you run or build the project, the tools pull those pieces together, compile the Dart code and produce the app. For newcomers, Google also offers an online editor called DartPad that runs small Dart snippets in the browser with nothing to install, which is a gentle way to try the language.

▶️ How to Run a DART File

Once the Dart tools are installed, running a .dart file from the command line is quick, and there are a couple of commands worth knowing depending on whether you want to run the code or turn it into a standalone program.

The everyday command is dart run, which executes a Dart program straight away. You can point it at a file, as in dart run app.dart, or run it with no filename inside a project folder, where it finds and runs the main program for you. Behind the scenes this compiles the code in the background and runs it, so the first run can take a moment. When you want a finished program rather than running the source each time, dart compile turns the code into an output you can distribute. Using dart compile exe produces a self-contained native executable that starts instantly and does not need the Dart tools installed to run, while dart compile js compiles the code into JavaScript for the web. One thing to note is that compiling is not cross-platform, so building an executable on Windows gives you a Windows program, and you build on each target system separately. To start a fresh project rather than a single file, dart create scaffolds a ready-made project folder for you to build on.

# run a Dart program right now
dart run app.dart

# build a standalone native executable
dart compile exe bin/app.dart -o app

# compile the code to JavaScript for the web
dart compile js bin/app.dart

# create a brand new project folder
dart create my_app

📦 Packages and pubspec.yaml

Real Dart projects almost never live in a single .dart file, and understanding how they are organised explains a file you will see everywhere in the Dart world, pubspec.yaml.

A Dart project is a package, which at its simplest is a folder containing a special file named pubspec.yaml. That file, written in the YAML format, holds the project's details such as its name and version, and importantly it lists the outside packages the project depends on. Those dependencies are reusable pieces of code published by others on pub.dev, the official package repository for Dart and Flutter, covering everything from making network requests to handling dates. To add one you run dart pub add followed by the package name, which records it in pubspec.yaml, and dart pub get downloads the listed packages so your code can use them. This is how a small project grows by standing on work others have shared rather than writing everything from scratch. So while a lone .dart file is just code, a proper project is a folder of .dart files alongside a pubspec.yaml that ties them to the packages they rely on.

📜 About the Dart Language

A little background on Dart itself helps explain why these files are so common today, since the language has an interesting history behind its recent surge in popularity.

Google first unveiled Dart in 2011 and released its first stable version in 2013, with the language created by Lars Bak and Kasper Lund. Early on it was pitched as a possible replacement for JavaScript, and there were plans to build a Dart engine into the Chrome browser, but Google dropped that idea in 2015 and Dart shifted its focus to compiling into JavaScript instead. For a while the language stayed fairly niche. Its fortunes changed dramatically with the rise of Flutter, which chose Dart as its language and carried it to a much wider audience of app developers. Dart is object-oriented and class-based with a familiar C-style syntax, and it has grown modern features over the years such as sound null safety, which helps prevent a common class of programming errors. It is also an open standard, recognised by the standards body Ecma, and remains free and open source. Today, if you meet a DART file, the overwhelming likelihood is that it belongs to a Flutter project.

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