Preventing BPF to move to next stage without satisfying the condition on the first stage

Hi All,

I have a scenario in the Business process flow to not to move to the next stage until it satisfies the condition in the current stage.

I have 5 stages and in each stage, I have a condition to meet. Once it is met, the BPF should allow it to move on to the next stage.  

The first BPF stage name is "verify Contact". 

This stage is having a field "verify Contact" in the BPF header. This has a drop-down list and when the user selects "Approved", it should get move to the next stage (or) else it should throw an error as  " Since Verify contact is not approved, you cannot move to next stage".

Can anyone please help me with how to do this in javascript?

please refer to the screenshot for more details.