Handling wrong authorities

src/lib/common/ErrorMessage.svelte

@@ -0,0 +1,8 @@
+<script lang="ts">
+  export let errorMessage: string;
+</script>
+
+<p class="govuk-error-message">
+  <span class="govuk-visually-hidden">Error:</span>
+  {errorMessage}
+</p>

src/lib/common/data_getter.ts

@@ -0,0 +1,18 @@
+import type { FeatureCollection } from "geojson";
+import authoritiesUrl from "../../../assets/authorities.geojson?url";
+
+export async function getAuthoritiesData() {
+  const resp = await fetch(authoritiesUrl);
+  const body = await resp.text();
+  const json: FeatureCollection = JSON.parse(body);
+  return Promise.resolve(json);
+}
+
+export async function getAuthoritiesNameSet() {
+  const json: FeatureCollection = await getAuthoritiesData();
+  const authoritySet: Set<string> = new Set();
+  for (let feature of json.features) {
+    authoritySet.add(feature.properties!.name);
+  }
+  return Promise.resolve(authoritySet);
+}

src/pages/App.svelte

@@ -5,6 +5,7 @@
   import authoritiesUrl from "../../assets/authorities.geojson?url";
   import BaselayerSwitcher from "../lib/BaselayerSwitcher.svelte";
   import BoundaryLayer from "../lib/BoundaryLayer.svelte";
+  import { getAuthoritiesNameSet } from "../lib/common/data_getter";
   import Layout from "../lib/common/Layout.svelte";
   import HoverLayer from "../lib/draw/HoverLayer.svelte";
   import InterventionLayer from "../lib/draw/InterventionLayer.svelte";
@@ -26,10 +27,10 @@
   let showInstructions = false;
 
   const params = new URLSearchParams(window.location.search);
-  // TODO Add validation and some kind of error page
   let authorityName: string = params.get("authority")!;
   let style: string = params.get("style") || "streets";
   let schema: Schema = (params.get("schema") as Schema) || "v1";
+  checkAuthorityValid(authorityName);
 
   // The version numbers here are arbitrary, not necessarily related to the
   // app's version. The version of the code deployed has to match the data, and
@@ -82,6 +83,14 @@
     );
     return geojson;
   }
+
+  async function checkAuthorityValid(authorityName: string): Promise<void> {
+    let authortiesNameSet = await getAuthoritiesNameSet();
+
+    if (!authortiesNameSet.has(authorityName)) {
+      window.location.href = `/?error=Authority name not found: ${authorityName}`;
+    }
+  }
 </script>
 
 <Layout>

src/pages/ChooseArea.svelte

@@ -4,18 +4,21 @@
   import { initAll } from "govuk-frontend";
   import { Map } from "maplibre-gl";
   import { onMount } from "svelte";
+  import ErrorMessage from "../lib/common/ErrorMessage.svelte";
   import DefaultButton from "../lib/govuk/DefaultButton.svelte";
   import FormElement from "../lib/govuk/FormElement.svelte";
   import Radio from "../lib/govuk/Radio.svelte";
   import SecondaryButton from "../lib/govuk/SecondaryButton.svelte";
   import "maplibre-gl/dist/maplibre-gl.css";
-  import authoritiesUrl from "../../assets/authorities.geojson?url";
+  import { getAuthoritiesData } from "../lib/common/data_getter";
   import FileInput from "../lib/common/FileInput.svelte";
   import About from "../lib/sidebar/About.svelte";
   import { bbox } from "../maplibre_helpers";
   import type { Schema } from "../types";
 
   let showAbout = false;
+  const params = new URLSearchParams(window.location.search);
+  let errorMessage: string = params.get("error")!;
 
   let inputValue: string;
   let dataList: HTMLDataListElement;
@@ -36,9 +39,7 @@
     // For govuk components. Must happen here.
     initAll();
 
-    const resp = await fetch(authoritiesUrl);
-    const body = await resp.text();
-    const json: FeatureCollection = JSON.parse(body);
+    const json: FeatureCollection = await getAuthoritiesData();
     for (let feature of json.features) {
       let option = document.createElement("option");
       option.value = feature.properties!.name;
@@ -157,6 +158,9 @@
     <SecondaryButton on:click={() => (showAbout = !showAbout)}
       >About</SecondaryButton
     >
+    {#if errorMessage}
+      <ErrorMessage bind:errorMessage />
+    {/if}
 
     <FormElement
       label="Select Transport Authority or Local Authority District"