feat: Gracefully handle non-English postcodes

app/api/route.ts

@@ -2,6 +2,7 @@ import { NextResponse } from "next/server";
 import { calculationSchema } from "../schemas/calculationSchema";
 import * as calculationService from "../services/calculationService";
 import calculateFairhold from "../models/testClasses";
+import { APIError } from "../lib/exceptions";
 
 export async function POST(req: Request) {
   try {
@@ -11,9 +12,22 @@ export async function POST(req: Request) {
     const householdData = await calculationService.getHouseholdData(input);
     const processedData = calculateFairhold(householdData);
     return NextResponse.json(processedData);
-  } catch (err) {
-    console.log("ERROR: API - ", (err as Error).message);
-    const response = { error: (err as Error).message };
+  } catch (error) {
+    console.log("ERROR: API - ", (error as Error).message);
+
+    if (error instanceof APIError) {
+      return NextResponse.json(
+        { error },
+        { status: error.status },
+      );
+    }
+
+    const response = {
+      error: { 
+        message: (error as Error).message, 
+        code: "UNHANDLED_EXCEPTION" 
+      },
+    };
     return NextResponse.json(response, { status: 500 });
   }
 }

app/components/ui/CalculatorInput.tsx

@@ -21,6 +21,7 @@ import {
 import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
 import { Label } from "@/components/ui/label";
+import { APIError } from "@/app/lib/exceptions";
 
 type View = "form" | "loading" | "dashboard";
 
@@ -68,10 +69,31 @@ const CalculatorInput = () => {
       body: JSON.stringify(data),
     });
     const processedData = await response.json();
+    if (!response.ok) return handleErrors(processedData.error);
+
     setData(processedData);
     setView("dashboard");
   };
 
+  const handleErrors = (error: APIError) => {
+    switch (error.code) {
+      case "ITL3_NOT_FOUND":
+      case "INSUFFICIENT_PRICES_PAID_DATA":
+        methods.setError(
+          "housePostcode", 
+          { message: 
+            "Insufficient data for this postcode. Please try again with a different postcode" 
+          }
+        );
+        break;
+      case "UNHANDLED_EXCEPTION":
+      default:
+        console.error(error)
+    };
+
+    setView("form");
+  }
+
   if (view === "form") {
     return (
       <div className="flex items-center justify-center text-black mt-5">

app/data/itlRepo.test.ts

@@ -46,7 +46,7 @@ describe("itlRepo", () => {
     await expect(
       itlRepo.getItl3ByPostcodeDistrict(postcodeDistrict)
     ).rejects.toThrow(
-      `Data error: Unable get get itl3 for postcode district ${postcodeDistrict}`
+      `Data error: Unable get itl3 for postcode district ${postcodeDistrict}`
     );
   });
 });

app/data/itlRepo.ts

@@ -1,4 +1,5 @@
 import prisma from "./db";
+import { APIError } from "../lib/exceptions";
 
 const getItl3ByPostcodeDistrict = async (
   postcodeDistrict: string
@@ -15,10 +16,11 @@ const getItl3ByPostcodeDistrict = async (
 
     return itl3;
   } catch (error) {
-    throw new Error(
-      `Data error: Unable get get itl3 for postcode district ${postcodeDistrict}`
-    );
-
+    throw new APIError({
+      status: 500,
+      message: `Data error: Unable get itl3 for postcode district ${postcodeDistrict}`,
+      code: "ITL3_NOT_FOUND"
+    });
   }
 };
 

app/data/pricesPaidRepo.ts

@@ -1,3 +1,4 @@
+import { APIError } from "../lib/exceptions";
 import prisma from "./db";
 
 const MINIMUM_NUMBER_OF_POSTCODES = 30;
@@ -32,7 +33,11 @@ const getPricesPaidByPostcodeAndHouseType = async (
     });
 
     if (!summary) {
-      throw new Error("Unable to find sufficient transaction data");
+    throw new APIError({
+      status: 500,
+      message: "Unable to find sufficient transaction data",
+      code: "INSUFFICIENT_PRICES_PAID_DATA",
+    });
     }
 
     return {

app/lib/exceptions.ts

@@ -0,0 +1,22 @@
+export type APIErrorCode =
+  | "ITL3_NOT_FOUND"
+  | "INSUFFICIENT_PRICES_PAID_DATA"
+  | "UNHANDLED_EXCEPTION";
+
+interface APIErrorParams {
+  code: APIErrorCode;
+  message: string;
+  status?: number;
+}
+
+export class APIError extends Error {
+  public code: APIErrorCode;
+  public status: number;
+
+  constructor({ code, message, status = 500 }: APIErrorParams) {
+    super(message);
+    this.name = "APIError";
+    this.code = code;
+    this.status = status;
+  }
+}

app/services/calculationService.ts

@@ -10,6 +10,7 @@ import { socialRentEarningsService } from "./socialRentEarningsService";
 import { rentService } from "./rentService"
 
 import { Calculation } from "../schemas/calculationSchema";
+import { APIError } from "../lib/exceptions";
 
 const prisma = new PrismaClient();
 
@@ -63,6 +64,8 @@ export const getHouseholdData = async (input: Calculation) => {
     };
 
   } catch (err) {
+    if (err instanceof APIError) throw err;
+
     throw Error(
       `Service error: Unable to generate household. Message: ${(err as Error).message}`
     );