🧪 Lesson 10: Testing Your Code

⏱ 30-40 min 📊 Intermediate 🧪 Quality Assurance 🔥 Professional 2026 🔗 Official Source

🧪

Why Testing Matters

Imagine building a bridge and never testing it. Scary, right? Testing your code is like quality control - it catches problems before your users do!

🛡️

Prevent Regressions

Tests catch bugs when you change old code

📋

Living Documentation

Tests show how code should behave

💪

Refactor Confidence

Change code fearlessly with a safety net

🚀

Faster Development

Less manual testing, more building

🎯

Today's Project: "Calculator Test Suite"

We'll build a comprehensive test suite for a calculator:

➕ Test basic arithmetic operations
🔄 Test edge cases (zero, negatives, overflow)
❌ Test error handling (division by zero)
⏱ Test async operations
📊 Organized with test groups

Step 1

Understanding Test Structure

🔍 Anatomy of a Test

import 'package:test/test.dart'; Bring in testing tools

group('Calculator', () { Organize related tests

test('adds two numbers', () { Define a single test case

expect(add(2, 3), equals(5)); Assert expected result

🖥️ Live Test Runner Simulator

Write a test and see it run:

📝 Test Code:

📤 Test Results:

$ dart test

00:00 +0: Loading tests...

Step 2

Matchers: The Expect Power Tools

🎯 Interactive Matcher Explorer

Select a matcher category and test values:

Value 1: Value 2:

equals() ✅ true

isNot(equals()) ❌ false

same() -

Value: Type:

isA<String>() -

isNull -

isNotNull -

List: Search item:

contains() -

hasLength() -

isEmpty -

String: Substring:

contains() -

startsWith() -

endsWith() -

Value: Compare to:

greaterThan() -

lessThan() -

inInclusiveRange() -

Code to test:

throwsException -

throwsFormatException -

returnsNormally -

Step 3

Organizing with Test Groups

🏗️ Test Group Hierarchy Builder

Build and visualize your test organization:

📁 Calculator

📁 addition

🧪 positive numbers

🧪 negative numbers

🧪 zero handling

📁 division

📄 Generated Test Structure:

group('Calculator', () {
  group('addition', () {
    test('positive numbers', () { ... });
    test('negative numbers', () { ... });
    test('zero handling', () { ... });
  });
  
  group('division', () {
    test('basic division', () { ... });
    test('division by zero', () { ... });
  });
});

Step 4

Testing Async Code

⏱ Async Test Simulator

See how different async test patterns work:

Pattern 1: async/await

test('fetchData returns value', () async {
  String result = await fetchData();
  expect(result, equals('data'));
});

Pattern 2: completion() matcher

test('fetchData completes with value', () {
  expect(
    fetchData(),
    completion(equals('data')),
  );
});

Pattern 3: throws with async

test('async operation throws', () async {
  expect(
    () async => await failingOp(),
    throwsException,
  );
});

Step 5

Testing JSON Models

📦 Model Test Generator

Generate tests for your JSON models automatically:

Model Fields (name:type:required):

Valid JSON:

🧪 Generated Tests:

group('User model', () {
  test('parses valid JSON correctly', () {
    final json = {'id': 1, 'name': 'Alice', 'email': 'alice@test.com', 'age': 30};
    final user = User.fromJson(json);
    expect(user.id, equals(1));
    expect(user.name, equals('Alice'));
    expect(user.email, equals('alice@test.com'));
    expect(user.age, equals(30));
  });

  test('handles missing optional fields', () {
    final json = {'id': 1, 'name': 'Alice'};
    final user = User.fromJson(json);
    expect(user.email, isNull);
    expect(user.age, isNull);
  });

  test('throws FormatException for invalid JSON', () {
    expect(
      () => User.fromJson({'wrong': 'format'}),
      throwsFormatException,
    );
  });
});

🏆

Build: Complete Calculator Test Suite

🔢 Calculator Implementation

🧪 Complete Test Suite

import 'package:test/test.dart';

void main() {
  final calc = Calculator();
  
  group('Calculator', () {
    group('addition', () {
      test('adds positive numbers', () {
        expect(calc.add(2, 3), equals(5));
        expect(calc.add(10, 20), equals(30));
      });
      
      test('handles negative numbers', () {
        expect(calc.add(-2, -3), equals(-5));
        expect(calc.add(-5, 10), equals(5));
      });
      
      test('handles zero', () {
        expect(calc.add(5, 0), equals(5));
        expect(calc.add(0, 0), equals(0));
      });
    });
    
    group('division', () {
      test('divides numbers correctly', () {
        expect(calc.divide(10, 2), equals(5.0));
        expect(calc.divide(7, 2), equals(3.5));
      });
      
      test('throws on division by zero', () {
        expect(
          () => calc.divide(10, 0),
          throwsA(isA<ArgumentError>()),
        );
      });
    });
    
    group('power', () {
      test('calculates powers correctly', () {
        expect(calc.power(2, 3), equals(8));
        expect(calc.power(5, 0), equals(1));
      });
      
      test('throws on negative exponent', () {
        expect(
          () => calc.power(2, -1),
          throwsArgumentError,
        );
      });
    });
    
    group('isPrime', () {
      test('identifies prime numbers', () {
        expect(calc.isPrime(2), isTrue);
        expect(calc.isPrime(7), isTrue);
        expect(calc.isPrime(13), isTrue);
      });
      
      test('identifies non-prime numbers', () {
        expect(calc.isPrime(1), isFalse);
        expect(calc.isPrime(4), isFalse);
        expect(calc.isPrime(100), isFalse);
      });
    });
    
    group('async operations', () {
      test('asyncAdd returns correct result', () async {
        final result = await calc.asyncAdd(3, 4);
        expect(result, equals(7));
      });
      
      test('asyncAdd completes with value', () {
        expect(
          calc.asyncAdd(5, 6),
          completion(equals(11)),
        );
      });
    });
  });
}

📊 Simulated Test Run

Click "Run Test Suite" to see results!

🎮 Testing Challenge: Quality Master Quiz

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⚡ Streak: 0

⏱ Time: 0s

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