buildArrayDiff(
    fieldName,
    oldArray : any[],
    newArray : any[],
    equalityFunction: (x, y) => boolean,
    printFunction : (x) =>  string) {

      let removed = oldArray.filter(x => !newArray.find(y => equalityFunction(x, y))).map(printFunction);
      let added = newArray.filter(x => !oldArray.find(y => equalityFunction(x, y))).map(printFunction);

      if (removed.length > 0 || added.length > 0) {
        this.fieldDiffs.push({
          fieldName : fieldName,
          removed : removed,
          added : added
        });
      }
  }

//example call

      this.buildArrayDiff(
        "Areas of Coverage",
        this.oldEntity.circulationData.areasOfCoverage,
        this.newEntity.circulationData.areasOfCoverage,
        (x : AreaOfCoverage, y : AreaOfCoverage) => {return x.geography.id == y.geography.id;},
        (x : AreaOfCoverage) => {return x.geography.displayString;});