Resource Allocation Check with JS

I would like some help with JS, as I am not familiar.

 

I have (field_112), which is Employee

I have (field_9), which is Project Name

I have (field_77), which is Start Date

I have (field_118), which is End Date

 

 

The Employee field allows for more than one Employee.

 

I would like a pop-up window, alert, or similar action to take place when a user tries to schedule an employee on a day when the employee is already scheduled between the start and end dates on a different project. 

 

I hope this makes sense. Thank you!