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const

Learn to define compile-time constants with const.

The const keyword

With const you declare a compile-time constant. The value is known before the program runs, during compilation. For example const maxScore = 100;.

const only works with constant expressions: numbers, string literals and operations on constants. Runtime-computed values like DateTime.now() cannot be const.

Because const values are computed at compile time, a single copy is kept in memory; this is a performance gain. Prefer const for values you know are fixed.

When to use const?

Typical cases for const: math constants (const pi = 3.14;), app versions (const version = '1.0.0';), fixed lists and configuration constants.

If a value must be computed at runtime (user input, time, random numbers) you cannot use const; choose final or var.

In frameworks like Flutter, writing const in front of const widgets is common; it boosts performance. We will see this later.

Practice Declare a const constant

Declare maxScore with const set to 100 and print it.

Starter code

main.dart
// Declare a const constant

Sample solution

main.dart
void main() {
  const maxScore = 100;
  print(maxScore);
}

Expected output

100

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Compile-time constant

Question 
____ maxScore = 100;
Answer 
const maxScore = 100;

Explanation: const is a compile-time constant.

2 What is the key difference between const and final?

  • final is faster
  • They are exactly the same
  • const can only be used inside a class
  • const is set at compile time, final at runtime Correct answer

Explanation: const at compile time, final at runtime.

3 const concepts

Term Meaning
const Compile-time constant
final Runtime constant
var Mutable
late Deferred init

All lessons in Beginner

1 Dart basics

  1. What is Dart?
  2. Your first program
  3. dart run
  4. Comments
  5. String interpolation
  6. Summary - Dart basics

2 Variables

  1. var and type
  2. final
  3. const
  4. late
  5. Null safety
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. bool and String
  4. Type inference
  5. dynamic
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Null-aware operators
  6. Summary - Operators

6 Strings

  1. Creating strings
  2. String interpolation
  3. length and methods
  4. substring
  5. contains and indexOf
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. switch
  4. Pattern matching
  5. Ternary
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. for-in
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Lists

  1. Creating lists
  2. List access
  3. add and remove
  4. length and contains
  5. List methods
  6. Summary - Lists

11 Sets and Maps

  1. Set
  2. Set operations
  3. Map
  4. Map operations
  5. Map iteration
  6. Summary - Sets and Maps

12 Functions

  1. Defining functions
  2. Return values
  3. Arrow functions
  4. Optional parameters
  5. First-class functions
  6. Summary - Functions

13 Null safety in depth

  1. Nullable types
  2. Null-aware operators
  3. The ! operator
  4. late
  5. Null check
  6. Summary - Null safety

14 Class basics

  1. class
  2. Constructor
  3. Instance variables
  4. Methods
  5. this
  6. Summary - Class basics

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string operations

  1. split and join
  2. replace
  3. trim and pad
  4. RegExp
  5. StringBuffer
  6. Summary - Advanced strings

17 Enums

  1. Defining enums
  2. Enum values
  3. Enhanced enums
  4. switch with enum
  5. Enum and patterns
  6. Summary - Enums

18 Exceptions

  1. try/catch
  2. throw
  3. finally
  4. The Exception class
  5. on clause
  6. Summary - Exceptions

19 Async basics

  1. Future
  2. async/await
  3. then
  4. Future.error
  5. Future.wait
  6. Summary - Async basics

20 Beginner final

  1. Project design
  2. Classes
  3. Collections
  4. Error handling
  5. Putting it together