mapJson is router parameter, which defines what notes field or view column will be returned in resulting json.
Syntax
mapJson 'notesFieldName/columnProgrammaticName', json:'jsonFieldName', type: 'STRING|INTEGER|DOUBLE|DATETIME|ARRAY_OF_STRING|ARRAY_OF_INTEGER|ARRAY_OF_DOUBLE|ARRAY_OF_DATETIME', isformula: true/false, formula: '', readonly: true/false, writeonly: true/false
notesFieldName (or columnProgrammaticName for VIEWENTRIES* strategies): Specifies what filed the data will be read from (or what view column)
json: specifies what key name it will have in the resulting JSON data
type: specifies the data type of the field/column, for multivalue fields/columns use ARRAY_OF_x
isformula: if true, value will be computed using formula specified in formula, default value is false (note: not applicable for VIEWENTRIES* strategies)
formula: specify formula, which will be executed in the context of current doc (note: not applicable for VIEWENTRIES* strategies)
readonly: if true, includes the field only in output
writeonly: if true: includes the field only when reading input JSON (works for POST/PUT methods only)