Introduction to Testing in Cadence

Testing is an essential part of smart contract development to ensure the correctness and reliability of your code. The Cadence Testing Framework provides a convenient way to write tests for your contracts, scripts and transactions which allows you to verify the functionality and correctness of your smart contracts.

Install Flow CLI

The Flow CLI is the primary tool for developing, testing, and deploying smart contracts to the Flow network.

If you haven't installed the Flow CLI yet and have homebrew installed, simply run brew install flow-cli. Alternatively, refer to the Flow CLI installation instructions.

Create a new project

In your preferred code editor, create a new directory for your project and navigate to it in the terminal. Then initialize a new Flow project by running the command flow init. This will create a flow.json config file that contains the project's configuration.

mkdir test-cadence
cd test-cadence
flow init

Write a simple smart contract

In your code editor, create a new file called calculator.cdc and add the following code:

calculator.cdc

access(all) contract Calculator {
    access(all)
    fun add(a: Int, b: Int): Int {
        return a + b
    }
    access(all)
    fun subtract(a: Int, b: Int): Int {
        return a - b
    }
    access(all)
    fun multiply(a: Int, b: Int): Int {
        return a * b
    }
}

Add the smart contract to the config

Next up, we need to add our new contract in the contracts key in the flow.json config file. More specifically, we need to add the contract name, location and an address alias for the testing environment.

{
  "contracts": {
    "Calculator": {
      "source": "./calculator.cdc",
      "aliases": {
        "testing": "0x0000000000000007"
      }
    }
  },
  "networks": {...},
  "accounts": {...},
  "deployments": {...}
}

For the time being, the address for the testing alias, can be one of:

0x0000000000000005
0x0000000000000006
0x0000000000000007
0x0000000000000008
0x0000000000000009
0x000000000000000a
0x000000000000000b
0x000000000000000c
0x000000000000000d
0x000000000000000e

In the next release, there will be 20 addresses for contract deployment during testing.

Write unit tests

In the same directory, create a new file called calculator_test.cdc and add the following code:

calculator_test.cdc

import Test
import "Calculator" // contract name from the previous step
access(all)
fun setup() {
    let err = Test.deployContract(
        name: "Calculator",
        path: "./calculator.cdc",
        arguments: []
    )
    Test.expect(err, Test.beNil())
}
access(all)
fun testAdd() {
    Test.assertEqual(5, Calculator.add(a: 2, b: 3))
}
access(all)
fun testSubtract() {
    Test.assertEqual(2, Calculator.subtract(a: 5, b: 3))
}

This code:

imports the Calculator contract from the calculator.cdc file (according to flow.json)
deploys the Calculator contract to the address specified in the testing alias
defines two test cases: testAdd() and testSubtract()
calls add() and subtract() methods with different input values respectively.

Running the test cases

To run the test cases, use the following command in the terminal:

flow test --cover --covercode="contracts" calculator_test.cdc

This command uses the Flow CLI to run the test cases and display the output. You should see the following output:

Test results: "calculator_test.cdc"
- PASS: testAdd
- PASS: testSubtract
Coverage: 66.7% of statements

This output indicates that both test cases ran successfully, and the two smart contract methods are functioning as expected. With the supplied flags (--cover & --covercode="contracts"), we also get code coverage insights for the contracts under testing. The code coverage percentage is 66.7%, because we have not added a test case for the multiply method. By viewing the auto-generated coverage.json file, we see:

{
  "coverage": {
    "A.0000000000000007.Calculator": {
      "line_hits": {
        "14": 0,
        "4": 1,
        "9": 1
      },
      "missed_lines": [
        14
      ],
      "statements": 3,
      "percentage": "66.7%"
    }
  }
}

Line 14 from the Calculator smart contract is marked as missed. This is the line:

return a * b

which is the multiply method.

By adding a test case for the above method:

calculator_test.cdc

...
access(all)
fun testMultiply() {
    Test.assertEqual(10, Calculator.multiply(a: 2, b: 5))
}

our code coverage percentage goes to 100%:

flow test --cover --covercode="contracts" calculator_test.cdc
Test results: "calculator_test.cdc"
- PASS: testAdd
- PASS: testSubtract
- PASS: testMultiply
Coverage: 100.0% of statements

Advanced Testing Techniques

The Cadence testing framework provides various features and techniques for writing comprehensive test scenarios. Some of these include:

Code Coverage: You can use the --cover flag with the flow test command to view code coverage results when running your tests. This allows you to identify areas of your code that are not adequately covered by your test inputs.
Test Helpers: Test helpers are reusable functions that help you set up the initial state for your test files. You can define test helpers in a Cadence program and use them in your test files by importing it whenever needed.
Assertions: The testing framework provides built-in assertion functions, such as assertEqual, beNil, beEmpty, contain, to help you verify the expected behavior of your smart contracts.
Test Suites: You can organize your test files into test suites to improve the readability and maintainability of your test code. Test suites allow you to group related test cases and set up common test helpers for all the tests in the suite.
Integration tests: In our previous example, we would directly call the available methods on the contract under test. This is generally categorized as unit testing. You can also write integration tests, by executing scripts & transactions to interact with the contracts under testing. If you would like to write your tests in Go, instead of Cadence, you can use Overflow tool to run integration tests against either an local emulator, testnet, mainnet or an in memory instance of the flow-emulator.

By leveraging these advanced testing techniques, you can write more robust and reliable smart contracts in Cadence. In this example, we set up a basic testing environment, wrote a simple smart contract in Cadence, and created a test file to verify its functionality. We then used the Flow CLI to run the test file and confirm that the smart contract is working correctly.

This is a basic example, and there are many more advanced features and techniques you can explore when working with the Cadence Testing Framework.

For more in-depth tutorials and documentation, refer to the official Cadence language documentation and the Flow CLI documentation.

Introduction to Testing in Cadence

Install Flow CLI​

Create a new project​

Write a simple smart contract​

Add the smart contract to the config​

Write unit tests​

Running the test cases​

Advanced Testing Techniques​

References​