How to calculate the key indicators of tasks, such as es, EF, LS, lf? Is there any API available ?
Unfortunately, there are no public API methods for that.
Gantt calculates that internally when autoscheduling functionality is enabled (with the constraints logic), and there is no way to use those internal methods.
Does that mean that this field already exists in the project and task properties? We don’t have to calculate, we can just show the columns, right?
No, there are no such fields:
Gantt has internal methods to calculate those values but they aren’t available as task properties or any other way.