Dart 101 🎯 Cheatsheet

A quick-reference for the Dart language, VM internals, and toolchain. No prose — just the facts.

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Language Basics

Types & Null Safety

Expression	Result
null is Object	false
null is Object?	true
1 is num	true
1.0 is int	false
List<int> is List<num>	true (covariant)
List<num> is List<int>	false

Null-Aware Operators

Op	Meaning
?.	Null-aware access
??	Null-coalescing
??=	Assign if null
!	Null assertion

Variables

Keyword	Reassignable	Immutability	Timing
var	Yes	—	Inferred
final	No	Runtime	Deferred ok
const	No	Compile-time	Must be const
late final	No	Runtime	First access

Falsy-like Values

Dart has no truthiness — only bool in conditions.

Value	if (x) valid
true / false	Yes
0, "", null	No (compile error)

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Functions

Feature	Syntax
Required positional	f(int a)
Optional positional	f(int a, [int b = 0])
Named	f({int a = 0})
Required named	f({required int a})
Arrow	int f(int x) => x * 2
Tear-off	final g = print
Typedef	typedef F = int Function(int)

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Collections

Literals

Literal	Type
[1, 2]	List<int>
{1, 2}	Set<int>
{'a': 1}	Map<String, int>
<int>[]	List<int>
{<String, int>{}}	Map<String, int>

List Methods

Method	Mutates	Returns
add	Yes	void
insert	Yes	void
remove	Yes	bool
sort	Yes	void
where / map	No	Iterable
reduce / fold	No	Value
firstWhere	No	T or throws
sublist	No	List
toList	No	List

Spread & Control Flow in Collections

Pattern	Example
Spread	[...a, ...b]
Null-aware spread	[...?maybeList]
If-element	[if (x) a]
For-element	[for (var x in xs) x * 2]

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Classes & OOP

Concept	Syntax
Constructor	Foo(this.x)
Named ctor	Foo.named()
Factory	factory Foo() => ...
Const ctor	const Foo(this.x)
Initializer list	Foo(int x) : _x = x
Getter	int get x => _x
Setter	set x(int v) => _x = v
Mixin	mixin M {} + with M
Extension	extension E on String {}
Sealed	sealed class S {}
Abstract	abstract class A {}

Access

Modifier	Scope
_name	Library-private
@protected	Subclass + library
@private	Library only (annotation)

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Async / Isolates

Futures & Streams

Construct	Purpose
Future<T>	Single async value
Stream<T>	Sequence of values
async	Returns Future<T>
await	Suspend until settled
async* / yield	Returns Stream<T>
await for	Iterate a Stream

Combinators

Method	Behavior
Future.wait	All resolve; any reject → reject
Future.any	First to complete
Future.delayed	Delayed future
Completer	Manual completion

Isolates

Concept	Detail
Memory	Independent per isolate
Comm	Message passing only
Isolate.spawn	Create isolate
Isolate.run	Spawn + return result
TransferableTypedData	Zero-copy transfer
dart:isolate on Wasm	Not available

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VM Internals — Object Layout

Component	Detail
tags_ field	GC bits + canonical bit + class ID
Class ID width	16 bits (concrete classes)
Class table	Per-isolate array → Class metadata
Smi tag	Low bit = 0
Heap object tag	Low bit = 1
Smi range (64-bit)	−2⁶² to 2⁶²−1
Smi range (32-bit)	−2³⁰ to 2³⁰−1

Smi vs Heap Number

Aspect	Smi	Heap Number
Allocates	No	Yes
GC-tracked	No	Yes
Write barrier	Eliminated	Required
Range	Tagged bits	Full 64-bit

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VM Internals — Inline Caches

IC States

State	Dispatch	Trigger
Unlinked	Resolution stub	First execution
Monomorphic	Class check + jump	One receiver class
Polymorphic	Linear scan of ICData	2..N classes
Megamorphic	Hash lookup	Exceeds threshold

ICData Format

Field	Purpose
numArgsChecked	Arguments checked
Class-target pairs	Dispatch groups
Terminator	End marker

Reload & Debug

Event	Effect
Hot reload	Resets mono/mega → poly/unlinked
Debugger stepping	Resets mono/mega → poly/unlinked

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VM Internals — GC & Safepoints

Space & Collector

Space	Collector	Algorithm
New	Scavenger	Copying (semispaces)
Old	Mark-sweep	Mark + sweep (concurrent)

Allocation Path

Condition	Action
Fits in TLAB	Bump allocate
Larger than TLAB	Old space
TLAB full	New buffer, GC if needed
Still fails	Old space

Thread States

State	At Safepoint
kThreadInGenerated	No
kThreadInVM	No
kThreadInNative	Yes
kThreadInBlockedState	Yes

Write Barrier

Barrier	Condition	Action
Generational	Old container + new value	Add to remembered set
Incremental	Old unmarked value + marking	Add to worklist

Barrier Elimination

Case	Eliminated
Constant	Yes
Smi	Yes
Self-reference	Yes
Fresh alloc, no GC	Yes

Safepoint Levels

Level	Requirements
GC	Stackmaps
Deopt	Stackmaps + deopt-id
Reload	Stackmaps + deopt-id + reload

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Compilation Targets

Target	Command	Output	Runtime
JIT	dart run	In-memory	VM
AOT	dart compile exe	Native binary	None (bundled)
JS	dart compile js	.js	Browser JS
Wasm	dart compile wasm	.wasm + .mjs	WasmGC

dart2js vs dart2wasm

Aspect	dart2js	dart2wasm
Backend	SSA → JS	WasmGC
GC	Browser JS	WasmGC
Output	Single .js	.wasm + loader
Null/undefined	Distinct	Collapsed
Browser req.	Any JS	WasmGC (Chrome 137+)

AOT Snapshot Format

Section	Contents	Deserialization
Data	Strings, metadata, types	Clustered, deserialized
Code	Machine code	Mapped directly
Instructions table	PC → stack map	Baked into rodata

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Async/Await Desugaring

Source	State Machine
async function	Class with moveNext
_state	Execution position
Completer	Returned Future
await	Register callback, return
Resumption	Callback calls moveNext
try/catch across await	_pendingError re-thrown in try
Local variables	Copied to SuspendState on suspend

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Toolchain

Command	Purpose
dart run main.dart	JIT execute
dart compile exe	Native binary
dart compile js	JavaScript
dart compile wasm	WebAssembly
dart analyze	Static analysis
dart format .	Format code
dart test	Run tests
dart pub get	Resolve deps
dart fix --apply	Apply fixes
dart doc .	Generate docs

pubspec.yaml Essentials

Section	Purpose
name	Package name
version	Semver
environment.sdk	SDK constraint
dependencies	Runtime deps
dev_dependencies	Dev-time deps
executables	CLI binaries

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Docker / Deployment

Multi-stage Build

FROM dart:stable AS build
WORKDIR /app
COPY pubspec.* ./
RUN dart pub get
COPY . .
RUN dart compile exe bin/server.dart -o bin/server

FROM scratch
COPY --from=build /runtime/ /
COPY --from=build /app/bin/server /bin/
CMD ["/bin/server"]

Runtime Base Comparison

Base	Size	Shell	HEALTHCHECK
scratch	~15 MB	No	Orchestrator only
alpine	~20 MB	Yes	Possible
debian:bookworm-slim	~90 MB	Yes	Yes

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Gotchas

Trap	Reality
typeof null	"object" in JS; Dart has no typeof — use is
NaN == NaN	false; use isNaN
Integer division	/ returns double; use ~/ for int
final list	List is immutable reference; contents mutable
const list	Contents are immutable
String interpolation	'$x' vs ${x + 1}
Cascade with ?	obj?..a()?..b()
late final re-init	Throws LateInitializationError
dynamic casts	No static check; runtime failure
Isolate sharing	Must transfer, not share

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Quick Decision Tables

var vs final vs const

Use when…	Keyword
Value reassigned	var
Value never reassigned, runtime	final
Value never reassigned, compile-time	const
Init deferred, never reassigned	late final

List vs Set vs Map

Need	Use
Ordered, indexed, duplicates	List
Unique values	Set
Key-value	Map

Future vs Stream

Need	Use
Single async result	Future
Multiple async results	Stream
Both	Stream<T>.first / Future<T>

Isolate vs Async

Need	Use
I/O bound	async/await
CPU bound	Isolate
Shared mutable state	Neither (message passing)

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One-Page Flow: Everything Connected

SOURCE
  │
  ├─ dart run ──────────────► JIT (dev, hot reload, profile)
  │
  ├─ dart compile exe ──────► AOT native (fast startup)
  │                               │
  │                               ├─ Docker: scratch / debian-slim
  │                               └─ No SDK needed at runtime
  │
  ├─ dart compile js ───────► JS (any browser)
  │
  └─ dart compile wasm ─────► WasmGC (modern browsers, dual build)

RUNTIME (VM)
  │
  ├─ Object header: tags_ (GC | canonical | class ID)
  ├─ Smi: encoded in pointer (low bit 0)
  ├─ Class table: class ID → vtable, metadata
  ├─ IC: unlinked → mono → poly → mega
  ├─ Alloc: TLAB bump → scavenge → promote → mark-sweep → sweep
  └─ Async: state machine (moveNext, _state, Completer)

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