Improve the typestest target in the Gulpfile

This commit:
- moves the preparation work to a new `typestest-pre` target similar to
  how the other targets work;
- moves the `TYPESTEST_DIR` definition to the top of the file like we
  did for all other directory variables;
- renames the `TYPES_BUILD_DIR` variable to `TYPES_DIR` since it's
  shorter and the naming scheme then corresponds to the other directory
  variables;
- switches to `const`/template strings in the types targets where needed;
- converts the `if (err !== null)` check to `if (err)` similar to other
  targets.

gulpfile.js

@@ -63,7 +63,8 @@ var GH_PAGES_DIR = BUILD_DIR + "gh-pages/";
 var SRC_DIR = "src/";
 var LIB_DIR = BUILD_DIR + "lib/";
 var DIST_DIR = BUILD_DIR + "dist/";
-var TYPES_BUILD_DIR = BUILD_DIR + "types/";
+var TYPES_DIR = BUILD_DIR + "types/";
+var TYPESTEST_DIR = BUILD_DIR + "typestest/";
 var COMMON_WEB_FILES = ["web/images/*.{png,svg,gif,cur}", "web/debugger.js"];
 var MOZCENTRAL_DIFF_FILE = "mozcentral.diff";
 
@@ -1143,12 +1144,12 @@ gulp.task("jsdoc", function (done) {
 });
 
 gulp.task("types", function (done) {
-  console.log("### Generating typescript definitions using tsc");
+  console.log("### Generating TypeScript definitions using `tsc`");
-  var args = [
+  const args = [
     "target ES2020",
     "allowJS",
     "declaration",
-    `outDir ${TYPES_BUILD_DIR}`,
+    `outDir ${TYPES_DIR}`,
     "strict",
     "esModuleInterop",
     "forceConsistentCasingInFileNames",
@@ -1353,16 +1354,9 @@ gulp.task(
 );
 
 gulp.task(
-  "typestest",
+  "typestest-pre",
-  gulp.series(
+  gulp.series("testing-pre", "generic", "types", function () {
-    "testing-pre",
+    const [packageJsonSrc] = packageBowerJson();
-    "generic",
-    "types",
-
-    function () {
-      var packageJsonSrc = packageBowerJson()[0];
-      var TYPESTEST_DIR = BUILD_DIR + "typestest/";
-
     return merge([
       packageJsonSrc.pipe(gulp.dest(TYPESTEST_DIR)),
       gulp
@@ -1372,20 +1366,21 @@ gulp.task(
           SRC_DIR + "pdf.worker.entry.js",
         ])
         .pipe(gulp.dest(TYPESTEST_DIR + "build/")),
-        gulp
+      gulp.src(TYPES_DIR + "**/**").pipe(gulp.dest(TYPESTEST_DIR + "build/")),
-          .src(TYPES_BUILD_DIR + "**/**")
-          .pipe(gulp.dest(TYPESTEST_DIR + "build/")),
     ]);
-    },
+  })
-    function (done) {
+);
-      exec(`node_modules/.bin/tsc -p test/types`, function (err, stdout) {
+
-        if (err !== null) {
+gulp.task(
-          console.log("couldn't compile typescript test: " + stdout);
+  "typestest",
+  gulp.series("typestest-pre", function (done) {
+    exec("node_modules/.bin/tsc -p test/types", function (err, stdout) {
+      if (err) {
+        console.log(`Couldn't compile TypeScript test: ${stdout}`);
       }
       done(err);
     });
-    }
+  })
-  )
 );
 
 gulp.task("baseline", function (done) {
@@ -1752,9 +1747,7 @@ gulp.task(
         gulp
           .src(LIB_DIR + "**/*", { base: LIB_DIR })
           .pipe(gulp.dest(DIST_DIR + "lib/")),
-        gulp
+        gulp.src(TYPES_DIR + "**/**").pipe(gulp.dest(DIST_DIR + "build/")),
-          .src(TYPES_BUILD_DIR + "**/**")
-          .pipe(gulp.dest(DIST_DIR + "build/")),
       ]);
     }
   )