Lesson 03 of 59

5 mins read

Variables, Type Inference, final & const

In Plain English

Dart variables can be declared explicitly (`String name`) or inferred statically with `var`. Immutability is enforced with two keywords: `final` (assigned once at runtime) and `const` (evaluated at compile time and deeply immutable). Understanding `final` vs `const` is fundamental to Flutter widget performance.

The Mental Model

"`var` is a label on a container whose contents can change; `final` is a locked box sealed once when packed; `const` is a bronze statue cast at the factory before shipping."

Deep Dive: How It Works

`var` Type Inference: Infers the exact static type on initialization (`var x = 10;` is statically typed as `int`).

`final`: A variable that can only be set once. Its value is computed at runtime (e.g. `final now = DateTime.now();`).

`const`: Compile-time constant. Canonicalized in memory (identical `const` objects share the exact same memory address).

Syntax Blueprint

var name = "Alice";      // Inferred static type (String)
final runtimeId = getUuid(); // Immutable after runtime assignment
const maxRetries = 5;       // Compile-time constant (Deeply immutable)

Three declaration styles balancing mutability, runtime evaluation, and compile-time canonicalization.

Core Rules to Remember

Static Type Inference: `var` is not dynamic; it locks in the inferred static type permanently.

final vs const: `final` is runtime single-assignment; `const` is compile-time immutable.

Memory Canonicalization: Identical `const` instances share a single memory address, saving RAM.

Common Beginner Traps & How to Fix Them

Mistake:Trying to assign a runtime function result to `const` (e.g. `const now = DateTime.now();`).

Why it happens: `DateTime.now()` cannot be evaluated until the app runs at runtime.

How to fix: Use `final now = DateTime.now();` for runtime initialization.

Pro Tip: In Flutter, always use `const` widget constructors wherever possible to skip widget rebuilds completely during state changes.

Live Interactive Example

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Comparing var, final, and const in Action

Python 3.12

Output Console

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How it works: Demonstrated mutable counter, runtime final piEstimate, and compile-time maxUsers.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Declare final and const Variables

Declare a `const String serverUrl = "https://api.zen.dev";`.
Declare a `final int port = 8080;`.
Print `Server: $serverUrl on port $port`.

Sandbox Output

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