The composer name consists of a vendor name before the slash and a name after the slash. To follow conventions, you should separate name parts with a dash.

The type must always be set to typo3-cms-extension

Meta information can be used to describe and classify your extension

The license must be GPL if you want to publish your extension to TER

Package links are specified in a require, require-dev, suggest and conflict sections, just like it can be done in the ext_emconf.php Regarding TYPO3 it is important to note, that specifying package names there is a bit tricky. For example when specifying a dependency to a PHP library which is available through composer, the TYPO3 extension manager will not be able to resolve this dependency and installation will fail. Especially when distributing the extension (e.g. through TER), it is important that no other dependencies are specified there except the one to TYPO3 itself. Extensions meant for distribution should specify dependencies in the ext_emconf.php file and only to other TER extensions. If the extension is meant for internal use and only ever installed via composer, it is fine to add any dependency to other composer libraries there, but only as of TYPO3 version 7.5.0. TYPO3 versions prior to that will complain about unresolved dependencies to unknown packages.

Please note, that you better specify the dependency to TYPO3 as "typo3/cms-core": "<version constraint your extension supports>" instead of using "typo3/cms": "<version constraint your extension supports>". Although you can specify the latter in TYPO3 versions higher than 7.5.0 or higher or equal 6.2.15, you very likely want to test against multiple TYPO3 versions anyway, so you can then also check if that works out (see test setup section below).

Composer allows packages to specify certain class auto loading information, so that, given the package is installed via composer, classes of the package can be loaded automatically without any additional require calls. TYPO3 currently also evaluates the psr-4 section in the composer.json Starting from TYPO3 version 7.5.0, the autoload section can also be specified in the ext_emconf.php file. Besides that it is recommended to register classes that are only needed to execute tests in the dedicated autoload-dev section. These classes will then be available, when doing composer install in the extension directory, to later execute the tests.

The replace section is used to specify packages, that are replaced when installing this package. It is also useful to define aliases of the same package. The latter is what is used by TYPO3 and the typo3/cms-composer-installers to determine the extension key of the package to be installed, so that a folder with the correct name can be created in typo3conf/ext/ Therefore it is mandatory for TYPO3 extensions with composer.json file to make use of this section and that the first replace has the extension key as name. As version, always use self.version which references the exact same version as the current package. A second line with the conventional name for TER extensions provided by https://composer.typo3.org is also useful but not mandatory.

It is possible to create an alias name for the branch of the VCS the composer.json file resides in. This is especially useful for the master branch. Read the composer documentation fur further information.

The rest is used to set up everything for your tests. Read the next chapter for details how to execute the tests.

The first thing you need to do is to go to your extension directory and "tell" composer to set up the testing environment for a TYPO3 version you want to test against. For that, type composer require typo3/cms=7.6.0 if e.g. TYPO3 7.6.0 is the TYPO3 version you want to test your extension with. This will will install TYPO3 and all other required dependencies in a (hidden) .Build/ folder. Since this command changed your composer.json file, make sure you reset the changes e.g. by doing a git checkout with git checkout composer.json.

If you want to use this test setup with TYPO3 6.2.x, you can try out this tweak, to get 6.2.x installed: composer require typo3/cms="~6.2.14" typo3/cms-composer-installers="1.2.2 as 1.1.4"

You can now execute the unit tests of your extension simply by calling phpunit with the following command:

TYPO3_PATH_WEB=$PWD/.Build/Web \
\
.Build/bin/phpunit --colors -c \
  .Build/vendor/typo3/cms/typo3/sysext/core/Build/UnitTests.xml \
  Tests/Unit

You can execute the functional tests of your extension simply by calling phpunit with the following command:

typo3DatabaseName="ext_scaffold" \
typo3DatabaseUsername="root" \
typo3DatabasePassword="root" \
typo3DatabaseHost="localhost" \
TYPO3_PATH_WEB=$PWD/.Build/Web \
\
.Build/bin/phpunit --colors -c \
  .Build/vendor/typo3/cms/typo3/sysext/core/Build/FunctionalTests.xml \
  Tests/Functional

Please note, that you need to specify a database user, that is allowed to create and delete databases for that to work. You can specify an arbitrary database name, it is only used to derive the test database names from.

The extension skeleton also comes with a .travis.yml file, which executes your tests (including a PHP syntax check) on every merge, by using the Travis CI service. For that, your git repository must be hosted on Github.