Fixed broken error decoder route

Author: Brian Vaughn

gatsby/createPages.js

@@ -59,7 +59,7 @@ module.exports = async (params) => {
     const {fields} = edge.node;
     let {id, language, languageCode, slug} = fields;
 
-    if (slug === 'docs/error-decoder.html') {
+    if (slug === '/docs/error-decoder.html') {
       // No-op so far as markdown templates go.
       // Error codes are managed by a page in src/pages
       // (which gets created by Gatsby during a separate phase).

gatsby/onCreatePage.js

@@ -12,9 +12,9 @@ module.exports = async ({page, boundActionCreators}) => {
   return new Promise(resolvePromise => {
     // page.matchPath is a special key that's used for matching pages only on the client.
     // Explicitly wire up all error code wildcard matches to redirect to the error code page.
-    if (page.path.includes('docs/error-decoder.html')) {
-      page.matchPath = 'docs/error-decoder:path?';
-      page.context.slug = 'docs/error-decoder.html';
+    if (page.path.includes('/docs/error-decoder.html')) {
+      page.matchPath = '/docs/error-decoder:path?';
+      page.context.slug = '/docs/error-decoder.html';
 
       createPage(page);
     }

src/components/ErrorDecoder/ErrorDecoder.js

@@ -39,9 +39,9 @@ function urlify(str: string): Node {
 // `?invariant=123&args[]=foo&args[]=bar`
 // or `// ?invariant=123&args[0]=foo&args[1]=bar`
 function parseQueryString(
-  search: string,
+  search: ?string,
 ): ?{|code: string, args: Array<string>|} {
-  const rawQueryString = search.substring(1);
+  const rawQueryString = search ? search.substring(1) : null;
   if (!rawQueryString) {
     return null;
   }
@@ -89,13 +89,15 @@ function ErrorResult(props: {|code: ?string, msg: string|}) {
 
 function ErrorDecoder(props: {|
   errorCodesString: string,
-  location: {search: string},
+  location: {search: ?string},
 |}) {
   let code = null;
   let msg = '';
 
-  const errorCodes = JSON.parse(props.errorCodesString);
-  const parseResult = parseQueryString(props.location.search);
+  const {errorCodesString, location} = props;
+
+  const errorCodes = JSON.parse(errorCodesString);
+  const parseResult = parseQueryString(location.search);
   if (parseResult != null) {
     code = parseResult.code;
     msg = replaceArgs(errorCodes[code], parseResult.args);

src/pages/docs/error-decoder.html.js

@@ -100,8 +100,8 @@ const ErrorPage = ({data, location}: Props) => (
 
 // eslint-disable-next-line no-undef
 export const pageQuery = graphql`
-  query ErrorPageMarkdown($slug: String!) {
-    markdownRemark(fields: {slug: {eq: $slug}}) {
+  query ErrorPageMarkdown {
+    markdownRemark(fields: {slug: {eq: "/docs/error-decoder.html"}}) {
       html
       fields {
         path