• Style ⟩

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:

1. 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.

2. 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

Style

Identifiers

• DO name types using UpperCamelCase.
• DO name extensions using UpperCamelCase.
• DO name libraries, packages, directories, and source files using lowercase_with_underscores.
• DO name import prefixes using lowercase_with_underscores.
• DO name other identifiers using lowerCamelCase.
• PREFER using lowerCamelCase for constant names.
• DO capitalize acronyms and abbreviations longer than two letters like words.
• DON’T use a leading underscore for identifiers that aren’t private.
• DON’T use prefix letters.

Ordering

• DO place “dart:” imports before other imports.
• DO place “package:” imports before relative imports.
• PREFER placing external “package:” imports before other imports.
• DO specify exports in a separate section after all imports.
• DO sort sections alphabetically.

Formatting

• DO format your code using dartfmt.
• CONSIDER changing your code to make it more formatter-friendly.
• AVOID lines longer than 80 characters.
• DO use curly braces for all flow control statements.

Documentation

Comments

• DO format comments like sentences.
• DON’T use block comments for documentation.

Doc comments

• DO use /// doc comments to document members and types.
• PREFER writing doc comments for public APIs.
• CONSIDER writing a library-level doc comment.
• CONSIDER writing doc comments for private APIs.
• DO start doc comments with a single-sentence summary.
• DO separate the first sentence of a doc comment into its own paragraph.
• AVOID redundancy with the surrounding context.
• PREFER starting function or method comments with third-person verbs.
• PREFER starting variable, getter, or setter comments with noun phrases.
• PREFER starting library or type comments with noun phrases.
• CONSIDER including code samples in doc comments.
• DO use square brackets in doc comments to refer to in-scope identifiers.
• DO use prose to explain parameters, return values, and exceptions.
• DO put doc comments before metadata annotations.

Markdown

• AVOID using markdown excessively.
• AVOID using HTML for formatting.
• PREFER backtick fences for code blocks.

Writing

• PREFER brevity.
• AVOID abbreviations and acronyms unless they are obvious.
• PREFER using “this” instead of “the” to refer to a member’s instance.

Usage

Libraries

• DO use strings in part of directives.
• DON’T import libraries that are inside the src directory of another package.
• PREFER relative paths when importing libraries within your own package’s lib directory.

Booleans

• DO use ?? to convert null to a boolean value.

Strings

• DO use adjacent strings to concatenate string literals.
• PREFER using interpolation to compose strings and values.
• AVOID using curly braces in interpolation when not needed.

Collections

• DO use collection literals when possible.
• DON’T use .length to see if a collection is empty.
• CONSIDER using higher-order methods to transform a sequence.
• AVOID using Iterable.forEach() with a function literal.
• DON’T use List.from() unless you intend to change the type of the result.
• DO use whereType() to filter a collection by type.
• DON’T use cast() when a nearby operation will do.
• AVOID using cast().

Functions

• DO use a function declaration to bind a function to a name.
• DON’T create a lambda when a tear-off will do.

Parameters

• DO use = to separate a named parameter from its default value.
• DON’T use an explicit default value of null.

Variables

• DON’T explicitly initialize variables to null.
• AVOID storing what you can calculate.

Members

• DON’T wrap a field in a getter and setter unnecessarily.
• PREFER using a final field to make a read-only property.
• CONSIDER using => for simple members.
• DON’T use this. except to redirect to a named constructor or to avoid shadowing.
• DO initialize fields at their declaration when possible.

Constructors

• DO use initializing formals when possible.
• DON’T type annotate initializing formals.
• DO use ; instead of {} for empty constructor bodies.
• DON’T use new.
• DON’T use const redundantly.

Error handling

• AVOID catches without on clauses.
• DON’T discard errors from catches without on clauses.
• DO throw objects that implement Error only for programmatic errors.
• DON’T explicitly catch Error or types that implement it.
• DO use rethrow to rethrow a caught exception.

Asynchrony

• PREFER async/await over using raw futures.
• DON’T use async when it has no useful effect.
• CONSIDER using higher-order methods to transform a stream.
• AVOID using Completer directly.
• DO test for Future<T> when disambiguating a FutureOr<T> whose type argument could be Object.

Design

Names

• DO use terms consistently.
• AVOID abbreviations.
• PREFER putting the most descriptive noun last.
• CONSIDER making the code read like a sentence.
• PREFER a noun phrase for a non-boolean property or variable.
• PREFER a non-imperative verb phrase for a boolean property or variable.
• CONSIDER omitting the verb for a named boolean parameter.
• PREFER the “positive” name for a boolean property or variable.
• PREFER an imperative verb phrase for a function or method whose main purpose is a side effect.
• PREFER a noun phrase or non-imperative verb phrase for a function or method if returning a value is its primary purpose.
• CONSIDER an imperative verb phrase for a function or method if you want to draw attention to the work it performs.
• AVOID starting a method name with get.
• PREFER naming a method to___() if it copies the object’s state to a new object.
• PREFER naming a method as___() if it returns a different representation backed by the original object.
• AVOID describing the parameters in the function’s or method’s name.
• DO follow existing mnemonic conventions when naming type parameters.

Libraries

• PREFER making declarations private.
• CONSIDER declaring multiple classes in the same library.

Classes and mixins

• AVOID defining a one-member abstract class when a simple function will do.
• AVOID defining a class that contains only static members.
• AVOID extending a class that isn’t intended to be subclassed.
• DO document if your class supports being extended.
• AVOID implementing a class that isn’t intended to be an interface.
• DO document if your class supports being used as an interface.
• DO use mixin to define a mixin type.
• AVOID mixing in a type that isn’t intended to be a mixin.

Constructors

• CONSIDER making your constructor const if the class supports it.

Members

• PREFER making fields and top-level variables final.
• DO use getters for operations that conceptually access properties.
• DO use setters for operations that conceptually change properties.
• DON’T define a setter without a corresponding getter.
• AVOID returning null from members whose return type is bool, double, int, or num.
• AVOID returning this from methods just to enable a fluent interface.

Types

• PREFER type annotating public fields and top-level variables if the type isn’t obvious.
• CONSIDER type annotating private fields and top-level variables if the type isn’t obvious.
• AVOID type annotating initialized local variables.
• AVOID annotating inferred parameter types on function expressions.
• AVOID redundant type arguments on generic invocations.
• DO annotate when Dart infers the wrong type.
• PREFER annotating with dynamic instead of letting inference fail.
• PREFER signatures in function type annotations.
• DON’T specify a return type for a setter.
• DON’T use the legacy typedef syntax.
• PREFER inline function types over typedefs.
• CONSIDER using function type syntax for parameters.
• DO annotate with Object instead of dynamic to indicate any object is allowed.
• DO use Future<void> as the return type of asynchronous members that do not produce values.
• AVOID using FutureOr<T> as a return type.

Parameters

• AVOID positional boolean parameters.
• AVOID optional positional parameters if the user may want to omit earlier parameters.
• AVOID mandatory parameters that accept a special “no argument” value.
• DO use inclusive start and exclusive end parameters to accept a range.

Equality

• DO override hashCode if you override ==.
• DO make your == operator obey the mathematical rules of equality.
• AVOID defining custom equality for mutable classes.

• DON’T check for null in custom == operators.

• Style ⟩