6 files changed, 56 insertions, 0 deletions
diff --git a/cli/js/deno.ts b/cli/js/deno.ts
index 887f60826..f5f774179 100644
--- a/cli/js/deno.ts
+++ b/cli/js/deno.ts
@@ -43,6 +43,7 @@ export {
   OpenOptions,
   OpenMode
 } from "./files.ts";
+export { formatDiagnostics } from "./format_error.ts";
 export { FsEvent, fsEvents } from "./fs_events.ts";
 export {
   EOF,
diff --git a/cli/js/dispatch.ts b/cli/js/dispatch.ts
index fc1c69a62..1b7e23463 100644
--- a/cli/js/dispatch.ts
+++ b/cli/js/dispatch.ts
@@ -17,6 +17,7 @@ export let OP_GET_DIR: number;
 export let OP_START: number;
 export let OP_APPLY_SOURCE_MAP: number;
 export let OP_FORMAT_ERROR: number;
+export let OP_FORMAT_DIAGNOSTIC: number;
 export let OP_CACHE: number;
 export let OP_RESOLVE_MODULES: number;
 export let OP_FETCH_ASSET: number;
diff --git a/cli/js/format_error.ts b/cli/js/format_error.ts
index d1d5a73f2..63250473b 100644
--- a/cli/js/format_error.ts
+++ b/cli/js/format_error.ts
@@ -1,4 +1,5 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+import { DiagnosticItem } from "./diagnostics.ts";
 import * as dispatch from "./dispatch.ts";
 import { sendSync } from "./dispatch_json.ts";
 
@@ -7,3 +8,11 @@ export function formatError(errString: string): string {
   const res = sendSync(dispatch.OP_FORMAT_ERROR, { error: errString });
   return res.error;
 }
+
+/**
+ * Format an array of diagnostic items and return them as a single string.
+ * @param items An array of diagnostic items to format
+ */
+export function formatDiagnostics(items: DiagnosticItem[]): string {
+  return sendSync(dispatch.OP_FORMAT_DIAGNOSTIC, { items });
+}
diff --git a/cli/js/format_error_test.ts b/cli/js/format_error_test.ts
new file mode 100644
index 000000000..282c2b274
--- /dev/null
+++ b/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();
+}
diff --git a/cli/js/lib.deno.ns.d.ts b/cli/js/lib.deno.ns.d.ts
index 0c9430869..4d7822c90 100644
--- a/cli/js/lib.deno.ns.d.ts
+++ b/cli/js/lib.deno.ns.d.ts
@@ -1903,6 +1903,13 @@ declare namespace Deno {
 
   /** UNSTABLE: new API, yet to be vetted.
    *
+   * Format an array of diagnostic items and return them as a single string.
+   * @param items An array of diagnostic items to format
+   */
+  export function formatDiagnostics(items: DiagnosticItem[]): string;
+
+  /** UNSTABLE: new API, yet to be vetted.
+   *
    * A specific subset TypeScript compiler options that can be supported by
    * the Deno TypeScript compiler.
    */
diff --git a/cli/js/unit_tests.ts b/cli/js/unit_tests.ts
index ec4505c21..1c8237466 100644
--- a/cli/js/unit_tests.ts
+++ b/cli/js/unit_tests.ts
@@ -23,6 +23,7 @@ import "./fetch_test.ts";
 import "./file_test.ts";
 import "./files_test.ts";
 import "./form_data_test.ts";
+import "./format_error_test.ts";
 import "./fs_events_test.ts";
 import "./get_random_values_test.ts";
 import "./globals_test.ts";