Collection Literals

Dart has rich literal syntax for creating collections inline. Beyond the basic [...], {...}, syntax, Dart supports collection if, collection for, the spread operator ..., and null-aware spread ...? — all directly inside literals.

Basic Literals

List Literal

// Inferred type: List<int>
var nums = [1, 2, 3, 4, 5];

// Explicit type
List<String> names = ['Alice', 'Bob', 'Carol'];
var typed = <double>[1.0, 2.5, 3.14];

// Empty list
var empty = <String>[];
List<int> emptyNums = [];

Set Literal

// Inferred type: Set<int>
var primes = {2, 3, 5, 7, 11};

// Explicit type
Set<String> fruits = {'apple', 'banana', 'cherry'};
var typed = <String>{'red', 'green', 'blue'};

// ⚠️ CRITICAL: empty {} is a Map, NOT a Set!
var emptyMap = {};        // Map<dynamic, dynamic>
var emptySet = <int>{};  // Set<int> ✅

Map Literal

// Inferred type: Map<String, int>
var scores = {'Alice': 90, 'Bob': 85, 'Carol': 92};

// Explicit type
Map<String, dynamic> config = {
  'host': 'localhost',
  'port': 8080,
  'debug': true,
};

// Empty map
var empty = <String, int>{};
Map<String, String> headers = {};

Type Inference Rules

var a = [1, 2, 3];        // List<int>
var b = [1, 2.5, 3];      // List<num>  (widened)
var c = [1, 'hello'];     // List<Object> (widened)
var d = <int>[1, 2, 3];   // List<int>  (explicit)

Spread Operator (`...`)

The spread operator inserts all elements of an iterable into a collection literal.

Spreading Lists

var a = [1, 2, 3];
var b = [4, 5, 6];

// Combine two lists
var combined = [...a, ...b];
print(combined); // [1, 2, 3, 4, 5, 6]

// Add elements around a spread
var extended = [0, ...a, ...b, 7, 8];
print(extended); // [0, 1, 2, 3, 4, 5, 6, 7, 8]

// Spread in the middle
var inserted = [1, 2, ...[10, 20, 30], 3, 4];
print(inserted); // [1, 2, 10, 20, 30, 3, 4]

Spreading into Sets

var setA = {1, 2, 3};
var setB = {3, 4, 5};

// Union via spread (creates a new Set)
var union = <int>{...setA, ...setB};
print(union); // {1, 2, 3, 4, 5}

Spreading into Maps

var defaults = {'color': 'blue', 'size': 'medium'};
var overrides = {'color': 'red', 'weight': 'bold'};

// Later spreads override earlier ones for duplicate keys
var merged = {...defaults, ...overrides};
print(merged); // {color: red, size: medium, weight: bold}

Shallow vs Deep Copy

var original = [1, 2, 3];
var copy = [...original];

copy.add(4);
print(original); // [1, 2, 3] — original unchanged

// NOTE: spread is a shallow copy
var nested = [[1, 2], [3, 4]];
var shallowCopy = [...nested];
shallowCopy[0].add(99); // mutates the inner list!
print(nested); // [[1, 2, 99], [3, 4]]

Null-Aware Spread (`...?`)

Spreads null safely — if the value is null, nothing is inserted.

List<int>? maybeList = null;

// ❌ Without null-aware: throws if null
// var bad = [0, ...maybeList, 9];

// ✅ With null-aware: null means "insert nothing"
var safe = [0, ...?maybeList, 9];
print(safe); // [0, 9]

// Conditional data from an API
List<String>? extraFeatures = fetchFeaturesOrNull();
var allFeatures = [
  'feature_a',
  'feature_b',
  ...?extraFeatures, // only added if not null
];

// Practical: optional items based on conditions
String? userRole = getUserRole(); // might be null

Map<String, dynamic> payload = {
  'action': 'update',
  'timestamp': DateTime.now().toIso8601String(),
  ...?(userRole != null ? {'role': userRole} : null),
};

`collection if`

Conditionally include elements in a collection literal.

Basic `if`

bool isLoggedIn = true;
bool isPremium = false;

var menuItems = [
  'Home',
  'Explore',
  if (isLoggedIn) 'Profile',
  if (isLoggedIn) 'Settings',
  if (isPremium) 'Premium Content',
  'Help',
];
print(menuItems);
// [Home, Explore, Profile, Settings, Help]

`if-else`

bool isDarkMode = true;

var themeConfig = {
  'mode': if (isDarkMode) 'dark' else 'light',
  'background': if (isDarkMode) '#1a1a1a' else '#ffffff',
  'text': if (isDarkMode) '#e2e8f0' else '#1a202c',
};

Nested `if`

bool hasAdmin = true;
bool hasModerator = false;
bool hasBasic = true;

var permissions = [
  if (hasAdmin) ...[
    'read', 'write', 'delete', 'admin',
  ] else if (hasModerator) ...[
    'read', 'write', 'moderate',
  ] else if (hasBasic) ...[
    'read',
  ],
];

`if` in Sets and Maps

bool showDebug = false;

Set<String> enabledFeatures = {
  'auth',
  'api',
  if (showDebug) 'debug_panel',
  if (showDebug) 'verbose_logging',
};

Map<String, dynamic> apiOptions = {
  'timeout': 30,
  'retries': 3,
  if (showDebug) 'logLevel': 'verbose',
  if (showDebug) 'traceRequests': true,
};

`collection for`

Generate elements using a for loop directly inside a collection literal.

Basic `for`

// Generate squares
var squares = [for (var i = 1; i <= 5; i++) i * i];
print(squares); // [1, 4, 9, 16, 25]

// Generate strings
var labels = [for (var i = 1; i <= 10; i++) 'Item $i'];
print(labels); // [Item 1, Item 2, ..., Item 10]

// Generate map entries
var indexMap = {for (var i = 0; i < 5; i++) i: 'value_$i'};
print(indexMap); // {0: value_0, 1: value_1, ..., 4: value_4}

`for-in`

var fruits = ['apple', 'banana', 'cherry'];

// Transform each element
var upperFruits = [for (var f in fruits) f.toUpperCase()];
print(upperFruits); // [APPLE, BANANA, CHERRY]

// Filter + transform
var longFruits = [
  for (var f in fruits)
    if (f.length > 5) f.toUpperCase(),
];
print(longFruits); // [BANANA, CHERRY]

// Map entries from a list
var fruitMap = {for (var f in fruits) f: f.length};
print(fruitMap); // {apple: 5, banana: 6, cherry: 6}

Nested `for`

// Multiplication table as a list of strings
var table = [
  for (var i = 1; i <= 3; i++)
    for (var j = 1; j <= 3; j++)
      '$i × $j = ${i * j}',
];
print(table);
// [1 × 1 = 1, 1 × 2 = 2, 1 × 3 = 3, 2 × 1 = 2, ...]

// 2D grid
var grid = [
  for (var row = 0; row < 3; row++)
    [for (var col = 0; col < 3; col++) row * 3 + col]
];
print(grid); // [[0,1,2], [3,4,5], [6,7,8]]

Combining Everything

// Complex real-world example: building a navigation menu
bool isLoggedIn = true;
bool isAdmin = false;
List<String> extraItems = ['Help', 'FAQ'];

var navigation = [
  'Home',
  'Products',
  if (isLoggedIn) ...[
    'Profile',
    'Orders',
    if (isAdmin) 'Admin Panel',
  ] else
    'Login',
  for (var item in extraItems) item,
  ...?getPromotionalItems(), // nullable spread
];

// Building a filter query map dynamically
String? nameFilter = 'Alice';
int? minAgeFilter = 18;
int? maxAgeFilter = null;
List<String>? roleFilter = ['admin', 'editor'];

var query = <String, dynamic>{
  'page': 1,
  'limit': 20,
  if (nameFilter != null) 'name': nameFilter,
  if (minAgeFilter != null) 'minAge': minAgeFilter,
  if (maxAgeFilter != null) 'maxAge': maxAgeFilter,
  ...?(roleFilter != null ? {'roles': roleFilter} : null),
};

Flutter Examples

@override
Widget build(BuildContext context) {
  return Column(
    children: [
      const Text('Welcome'),
      if (isLoggedIn) ...[
        UserProfile(user: currentUser),
        const Divider(),
      ],
      for (final item in menuItems)
        ListTile(
          title: Text(item.title),
          onTap: () => navigate(item.route),
        ),
      if (!isLoggedIn) const LoginButton(),
    ],
  );
}

Dynamic Tab Items

List<BottomNavigationBarItem> buildNavItems() => [
  const BottomNavigationBarItem(icon: Icon(Icons.home), label: 'Home'),
  const BottomNavigationBarItem(icon: Icon(Icons.explore), label: 'Explore'),
  if (user.isPremium)
    const BottomNavigationBarItem(icon: Icon(Icons.star), label: 'Premium'),
  const BottomNavigationBarItem(icon: Icon(Icons.person), label: 'Profile'),
];

GridView with Collection For

GridView.count(
  crossAxisCount: 3,
  children: [
    for (final color in Colors.primaries)
      Container(
        color: color,
        child: Center(child: Text(color.toString())),
      ),
  ],
)

Common Mistakes

❌ Forgetting `{}` is a Map, not a Set

var empty = {};      // ❌ Map<dynamic, dynamic>
var emptySet = <int>{}; // ✅ Set<int>

❌ Spread doesn't deep-copy nested collections

var original = [[1, 2], [3, 4]];
var copy = [...original]; // shallow!
copy[0].add(99);
print(original[0]); // [1, 2, 99] — mutated!

// ✅ Deep copy
var deep = [for (var list in original) [...list]];

❌ `collection if` without braces for multi-line expressions

// ❌ This only conditionally includes 'b', not 'a'
var list = [
  if (condition)
    'a',
    'b', // always included — not part of the if!
];

// ✅ Use spread + list for multiple items
var list = [
  if (condition) ...['a', 'b'],
];

Best Practices

Use spread ... instead of ..addAll() for combining collections.
Use collection if instead of building lists with if statements outside the literal.
Use collection for instead of List.generate for readable transformations.
Use ...? (null-aware spread) instead of a null check before spreading.
Use const literals for compile-time constant collections.

Basic Literals​

List Literal​

Set Literal​

Map Literal​

Type Inference Rules​

Spread Operator (...)​

Spreading Lists​

Spreading into Sets​

Spreading into Maps​

Shallow vs Deep Copy​

Null-Aware Spread (...?)​

collection if​

Basic if​

if-else​

Nested if​

if in Sets and Maps​

collection for​

Basic for​

for-in​

Nested for​

Combining Everything​

Flutter Examples​

Dynamic Widget List​

Dynamic Tab Items​

GridView with Collection For​

Common Mistakes​

❌ Forgetting {} is a Map, not a Set​

❌ Spread doesn't deep-copy nested collections​

❌ collection if without braces for multi-line expressions​

Best Practices​