feat: Add Deno.formatDiagnostics (#4032)

cli/js/format_error_test.ts

@@ -0,0 +1,37 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+import { assert, test } from "./test_util.ts";
+
+test(function formatDiagnosticBasic() {
+  const fixture: Deno.DiagnosticItem[] = [
+    {
+      message: "Example error",
+      category: Deno.DiagnosticCategory.Error,
+      sourceLine: "abcdefghijklmnopqrstuv",
+      lineNumber: 1000,
+      scriptResourceName: "foo.ts",
+      startColumn: 1,
+      endColumn: 2,
+      code: 4000
+    }
+  ];
+  const out = Deno.formatDiagnostics(fixture);
+  assert(out.includes("Example error"));
+  assert(out.includes("foo.ts"));
+});
+
+test(function formatDiagnosticError() {
+  let thrown = false;
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  const bad = ([{ hello: 123 }] as any) as Deno.DiagnosticItem[];
+  try {
+    Deno.formatDiagnostics(bad);
+  } catch (e) {
+    assert(e instanceof TypeError);
+    thrown = true;
+  }
+  assert(thrown);
+});
+
+if (import.meta.main) {
+  Deno.runTests();
+}