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Type inference

Learn var and type inference.

Type inference

Type inference means Dart automatically determines a variable's type from the value on the right. It works with the var keyword.

For example var age = 25; makes Dart infer age is an int. var price = 9.99; is a double, var name = 'Alex'; is a String.

This speeds up writing code. But once the type is set, it cannot change: var x = 5; followed by x = 'text'; is an error.

Practice Three types with var

Inside void main() declare var age = 25, var price = 9.99 and var name = 'Alex'. Print all three with print.

Starter code

main.dart
// Declare variables with var

Sample solution

main.dart
void main() {
  var age = 25;
  var price = 9.99;
  var name = 'Alex';
  print(age);
  print(price);
  print(name);
}

Expected output

25
9.99
Alex

Limits of inference

Type inference is powerful but does not fit every case. For example in function parameters and class fields, writing the explicit type is common; it improves readability.

Also null cannot be the initial value: var x = null; makes Dart unable to infer and it assumes dynamic. This is undesirable; using a nullable type is better.

Tip: if the type is clearly known, write it; if unclear, use var. Restraint is always good.

Questions and answers

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

1 When you write var x = 3.14;, what is the type of x?

main.dart
var x = 3.14;
  • int
  • dynamic
  • double Correct answer
  • object

Explanation: 3.14 is decimal so x becomes a double.

2 Inference keyword

Question 
____ name = 'Alex';
Answer 
var name = 'Alex';

Explanation: var triggers type inference.

3 Inference concepts

Term Meaning
var Inference
int Integer
double Floating-point
String Text

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