Truffle's JavaScript testing framework is built on top of mocha. Mocha is a JavaScript framework to write tests, whereas chai is an assertion library.

Testing frameworks are used to organize and execute tests, whereas assertion libraries provide utilities to verify that things are correct. Assertion libraries make it a lot easier to test your code so you don't have to perform thousands of if statements. Most of the testing frameworks don't have an assertion library included and let the user plug which one they want to use.

Your tests should exist in the ./test directory, and they should end with a .js extension.

Contract abstractions are the basis for making contract interaction possible from JavaScript. Because truffle has no way of detecting which contracts you'll need to interact with within your tests, you'll need to ask for these contracts explicitly. You do this by using the artifacts.require() method. So the first thing that should be done in test files is to create abstractions for the contracts that you want to test.

Then, the actual tests should be written. Structurally, your tests should remain largely unchanged from those of mocha. The test files should contain code that mocha will recognize as an automated test. What makes truffle tests different from mocha is the contract() function: this function works exactly like describe(), except that it signals truffle to run all migrations. The contract() function works like this:

• Before each contract() function is run, your contracts are redeployed to the running ethereum node, so the tests within it run with a clean contract state
• The contract() function provides a list of accounts made available by your ethereum node, which you can use to write tests

Here is the default test code generated by truffle to test the MetaCoin contract. You will find this code in the metacoin.js file:

// Specifically request an abstraction for MetaCoin.sol 
var MetaCoin = artifacts.require("./MetaCoin.sol"); 

contract('MetaCoin', function(accounts) { 
  it("should put 10000 MetaCoin in the first account", function() { 
    return MetaCoin.deployed().then(function(instance) { 
      return instance.getBalance.call(accounts[0]); 
    }).then(function(balance) { 
      assert.equal(balance.valueOf(), 10000, "10000 wasn't in the first account"); 
    }); 
  }); 
  it("should send coin correctly", function() { 
    var meta; 

    // Get initial balances of first and second account. 
    var account_one = accounts[0]; 
    var account_two = accounts[1]; 

    var account_one_starting_balance; 
    var account_two_starting_balance; 
    var account_one_ending_balance; 
    var account_two_ending_balance; 

    var amount = 10; 

    return MetaCoin.deployed().then(function(instance) { 
      meta = instance; 
      return meta.getBalance.call(account_one); 
    }).then(function(balance) { 
      account_one_starting_balance = balance.toNumber(); 
      return meta.getBalance.call(account_two); 
    }).then(function(balance) { 
      account_two_starting_balance = balance.toNumber(); 
      return meta.sendCoin(account_two, amount, {from: account_one}); 
    }).then(function() { 
      return meta.getBalance.call(account_one); 
    }).then(function(balance) { 
      account_one_ending_balance = balance.toNumber(); 
      return meta.getBalance.call(account_two); 
    }).then(function(balance) { 
      account_two_ending_balance = balance.toNumber(); 

      assert.equal(account_one_ending_balance, account_one_starting_balance - amount, "Amount wasn't correctly taken from the sender"); 
      assert.equal(account_two_ending_balance, account_two_starting_balance + amount, "Amount wasn't correctly sent to the receiver"); 
    }); 
  }); 
});

In the preceding code, you can see that all the contract's interaction code is written using the truffle-contract library. The code is self-explanatory.

Finally, truffle gives you access to mocha's configuration so you can change how mocha behaves. mocha's configuration is placed under a mocha property in the truffle.js file's exported object. For example, take a look at this:

mocha: { 
  useColors: true 
}