Set the field value of one entity to another entity

I have 2 entities A and B with no entity relation between them . In the B entity i have a single line of text field .if i enter the value of the field in entity B it should compare with the field in entity A .if the value  matches it should set the value to another field of entity B .I am trying to do with JavaScript .but i don't know how to start .can anyone help on this

Thanks,