Dart Type Inference

Let Dart automatically determine variable types

🔍 What is Type Inference?

Type inference allows Dart to automatically determine variable types from their values. Use 'var', 'final', or 'const' keywords to let Dart figure out types. 


// Dart infers the type automatically
var name = 'Alice';  // String
var age = 25;        // int
var height = 5.9;    // double

Type Inference Keywords

🔄

var

Mutable variables with inferred type 

var count = 10;
count = 20; // Can change
🔒

final

Immutable variables set at runtime 

final time = DateTime.now();
// time = something; // Error!
💎

const

Compile-time constants 

const pi = 3.14159;
const greeting = 'Hello';
📝

Explicit Types

Manually specify types 

String name = 'Bob';
int score = 95;

🔹 Using var Keyword

Let Dart infer types for mutable variables: 

// Dart automatically determines types
var message = 'Hello World';    // String
var count = 42;                 // int
var price = 19.99;              // double
var isActive = true;            // bool
var items = ['apple', 'banana']; // List

// You can change values but not types
count = 100;        // OK - still int
// count = 'text';  // Error - can't change type

Output:

message: Hello World (String)

count: 100 (int)

price: 19.99 (double)

🔹 Using final Keyword

Create immutable variables with inferred types: 

// Set once, can't change later
final userName = 'alice123';
final currentTime = DateTime.now();
final randomNumber = 42;

print('User: $userName');
print('Time: $currentTime');

// This would cause an error:
// userName = 'bob456'; // Error!

Output:

User: alice123

Time: 2024-01-15 10:30:45.123

🔹 Using const Keyword

Create compile-time constants: 

// Known at compile time
const appName = 'My Dart App';
const version = 1.0;
const maxUsers = 1000;
const features = ['login', 'dashboard', 'settings'];

// Mathematical expressions
const area = 3.14 * 5 * 5;  // Calculated at compile time

print('$appName v$version');
print('Max users: $maxUsers');

Output:

My Dart App v1.0

Max users: 1000

🔹 When to Use Each

Choose the right keyword for your needs:

Use var when:

  • Variable value will change
  • Type is obvious from the value
  • You want cleaner, shorter code

Use final when:

  • Value is set once and never changes
  • Value is determined at runtime
  • You want to prevent accidental changes

Use const when:

  • Value is known at compile time
  • Creating truly immutable objects
  • Performance optimization is important 
// Examples of when to use each
var counter = 0;              // Will change
final userId = getUserId();   // Set once at runtime  
const appTitle = 'My App';    // Never changes, known at compile time

🧠 Test Your Knowledge

Which keyword should you use for a value that never changes and is known at compile time?