Ansible playbook is just a list of tasks executed one by one in the order you define them in the playbook code. Using the conditional statements you may skip execution of some tasks, but there is one general rule that should apply to all playbooks you create – keep the number of tasks at the minimum.
Execution of each task takes time, and until you provide additional optimization, Ansible will establish the connection, perform an authentication process and then terminate the connection with a remote device. The longer this process takes, the more time you waste when you execute the playbook. It is a good practice to consolidate the tasks – if you need to perform multiple commands on a remote network device, you should define them as a parameter of one task instead of running them in separate tasks. In rare cases it may lead to unexpected problems like the one I described in my post Automated scripts can send commands faster than RP can process.
Sometimes you need to vary the value of an option provided to the task module. If you need to get an output of a CLI command on Juniper device, you will use the module junos_command which is part of standard Ansible library. Using the display parameter, you can specify if command output will be encoded in XML or JSON format. JSON is a more flexible format, but it is not a supported output format on older JunOS version. If you try to request it but the firmware does not support it your task, and the whole playbook will fail. Most of the developers will create two separate task and the conditional test to check version as a task with the when option. However, let me show you the other, not that well known, way.