Duplicate and editable fields

Hello, community!

I don't have the coding background and am trying to build a fillable form with some javascript. I am stuck where I have two fields, Field_A and Field_B. When I fill out Field_A, Field_B will automatically duplicate the same info. However, occasionally, Field_B will need to be different than Field_A (manually edited). The fields contain various characters (-1.50, +2.00, abcdf, etc.). 

This is what I am using in the Field_B custom calculation script, and it seems to work for duplication and editable, but once I edit different fields on the form, Field_B would jump back to the original value from Field_A.

event.target.value = this.getField("Field_A").valueAsString;

Thanks for the help.