Effective Dart

Over the past several years, we’ve written a ton of Dart code and learned a lot about what works well and what doesn’t. We’re sharing this with you so you can write consistent, robust, fast code too. There are two overarching themes:

Be consistent. When it comes to things like formatting, and casing, arguments about which is better are subjective and impossible to resolve. What we do know is that being consistent is objectively helpful. If two pieces of code look different it should be because they are different in some meaningful way. When a bit of code stands out and catches your eye, it should do so for a useful reason. Be brief. Dart was designed to be familiar, so it inherits many of the same statements and expressions as C, Java, JavaScript and other languages. But we created Dart because there is a lot of room to improve on what those languages offer. We added a bunch of features, from string interpolation to initializing formals, to help you express your intent more simply and easily. If there are multiple ways to say something, you should generally pick the most concise one. This is not to say you should code golf yourself into cramming a whole program into a single line. The goal is code that is economical, not dense.

The Dart analyzer has a linter to help you write good, consistent code. If a linter rule exists that can help you follow a guideline, then the guideline links to that rule. Here’s an example:

Linter rule: prefer_collection_literals

For help on enabling linter rules, see the documentation for customizing static analysis.

The guides

We split the guidelines into a few separate pages for easy digestion:

Style Guide – This defines the rules for laying out and organizing code, or at least the parts that dartfmt doesn’t handle for you. The style guide also specifies how identifiers are formatted: camelCase , using_underscores , etc.

Documentation Guide – This tells you everything you need to know about what goes inside comments. Both doc comments and regular, run-of-the-mill code comments.

Usage Guide – This teaches you how to make the best use of language features to implement behavior. If it’s in a statement or expression, it’s covered here.

Design Guide – This is the softest guide, but the one with the widest scope. It covers what we’ve learned about designing consistent, usable APIs for libraries. If it’s in a type signature or declaration, this goes over it.

For links to all the guidelines, see the summary.

How to read the guides

Each guide is broken into a few sections. Sections contain a list of guidelines. Each guideline starts with one of these words:

DO guidelines describe practices that should always be followed. There will almost never be a valid reason to stray from them.

DON’T guidelines are the converse: things that are almost never a good idea. Hopefully, we don’t have as many of these as other languages do because we have less historical baggage.

PREFER guidelines are practices that you should follow. However, there may be circumstances where it makes sense to do otherwise. Just make sure you understand the full implications of ignoring the guideline when you do.

AVOID guidelines are the dual to “prefer”: stuff you shouldn’t do but where there may be good reasons to on rare occasions.

CONSIDER guidelines are practices that you might or might not want to follow, depending on circumstances, precedents, and your own preference.

Some guidelines describe an exception where the rule does not apply. When listed, the exceptions may not be exhaustive—you might still need to use your judgement on other cases.

This sounds like the police are going to beat down your door if you don’t have your laces tied correctly. Things aren’t that bad. Most of the guidelines here are common sense and we’re all reasonable people. The goal, as always, is nice, readable and maintainable code.

Glossary

To keep the guidelines brief, we use a few shorthand terms to refer to different Dart constructs.

A library member is a top-level field, getter, setter, or function. Basically, anything at the top level that isn’t a type.

A class member is a constructor, field, getter, setter, function, or operator declared inside a class. Class members can be instance or static, abstract or concrete.

A member is either a library member or a class member.

A variable , when used generally, refers to top-level variables, parameters, and local variables. It doesn’t include static or instance fields.

A type is any named type declaration: a class, typedef, or enum.

A property is a top-level variable, getter (inside a class or at the top level, instance or static), setter (same), or field (instance or static). Roughly any “field-like” named construct.

Summary of all rules