The API documentation for OpenLayers 3.11.0, the version we are using, can be found at http://www.openlayers.org/en/v3.11.0/apidoc/. The API docs, like the library itself, are versioned, thus you can browse the appropriate documentation for your OpenLayers 3 version by changing v3.11.0 in the URL to the version you are currently using.
Note
The development version of the API is also documented; you can always reach it at http://www.openlayers.org/en/master/apidoc/. Be careful when you use it, though. It contains all of the newly implemented methods, which probably won't work with the latest stable version.
Check the API documentation by typing one of the preceding links in your browser. You should see the home page with the most frequently used classes. There is also a handy search box, with all of the classes listed on the left side. We have talked about default interactions and their lengthy nature before. On the home page you can see a link to the default interactions. If you click on it, you will be directed to the following page:
Now you can also see that nine interactions are added to the map by default. It would be quite verbose to add them one by one just to keep them when we define only one extra interaction, wouldn't it?
Note
You can see some features marked as experimental while you browse the API documentation with the Stable Only checkbox unchecked. Do not consider those features to be unreliable. They are stable, but experimental, and therefore they can be modified or removed in future versions. If the developer team considers a feature is useful and does not need further optimization or refactoring, it will be marked as stable.
For every constructor and function in the API, the input and expected output types are well documented. To see a good example, let's search for a function with inputs and outputs as well. If you search for ol.proj.fromLonLat, you will see the following function:
The function takes two arguments as input, one named coordinate and one named projection; projection is optional. coordinate is an ol.Coordinate type (an array with two numbers), while projection is an ol.proj.ProjectionLike type (a string representing the projection). The returned value, as we can see next to the white arrow, is also an ol.Coordinate type, with the transformed values.
Tip
A good developer always keeps track of future changes in the library. This is especially important with OpenLayers 3 when a major change occurs, as it lacks backward compatibility. You can see all of the major changes in the library in the OpenLayers 3 GitHub repository: https://github.com/openlayers/ol3/blob/master/changelog/upgrade-notes.md.