manual: fix a syntax error in redirects.js that made it not do anything

lol lmao

Let's put in a syntax checker in CI so we do not have to deal with this
nonsense ever again.

Change-Id: I0fe875e0cfc59ab1783087762e5bb07e09ded105

doc/manual/redirects.js

@@ -345,7 +345,7 @@ const redirects = {
     "linux": "uninstall.html#linux",
     "macos": "uninstall.html#macos",
     "uninstalling": "uninstall.html",
-  }
+  },
   "contributing/hacking.html": {
     "nix-with-flakes": "#building-nix-with-flakes",
     "classic-nix": "#building-nix",

flake.nix

@@ -292,6 +292,24 @@
             werror = true;
           };
 
+          # Although this might be nicer to do with pre-commit, that would
+          # require adding 12MB of nodejs to the dev shell, whereas building it
+          # in CI with Nix avoids that at a cost of slower feedback on rarely
+          # touched files.
+          jsSyntaxCheck =
+            let
+              nixpkgs = nixpkgsFor.x86_64-linux.native;
+              inherit (nixpkgs) pkgs;
+              docSources = lib.fileset.toSource {
+                root = ./doc;
+                fileset = lib.fileset.fileFilter (f: f.hasExt "js") ./doc;
+              };
+            in
+            pkgs.runCommand "js-syntax-check" { } ''
+              find ${docSources} -type f -print -exec ${pkgs.nodejs-slim}/bin/node --check '{}' ';'
+              touch $out
+            '';
+
           # Make sure that nix-env still produces the exact same result
           # on a particular version of Nixpkgs.
           evalNixpkgs =