diff --git a/cli/diagnostics.rs b/cli/diagnostics.rs
index 104a72c909..aaa6a12275 100644
--- a/cli/diagnostics.rs
+++ b/cli/diagnostics.rs
@@ -1,6 +1,10 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
 //! This module encodes TypeScript errors (diagnostics) into Rust structs and
 //! contains code for printing them to the console.
+
+// TODO(ry) This module does a lot of JSON parsing manually. It should use
+// serde_json.
+
 use crate::colors;
 use crate::fmt_errors::format_maybe_source_line;
 use crate::fmt_errors::format_maybe_source_name;
@@ -29,7 +33,7 @@ impl Diagnostic {
       let items_values = items_v.as_array().unwrap();
 
       for item_v in items_values {
-        items.push(DiagnosticItem::from_json_value(item_v));
+        items.push(DiagnosticItem::from_json_value(item_v)?);
       }
     }
 
@@ -114,14 +118,13 @@ pub struct DiagnosticItem {
 }
 
 impl DiagnosticItem {
-  pub fn from_json_value(v: &serde_json::Value) -> Self {
+  pub fn from_json_value(v: &serde_json::Value) -> Option<Self> {
     let obj = v.as_object().unwrap();
 
     // required attributes
     let message = obj
       .get("message")
-      .and_then(|v| v.as_str().map(String::from))
-      .unwrap();
+      .and_then(|v| v.as_str().map(String::from))?;
     let category = DiagnosticCategory::from(
       obj.get("category").and_then(Value::as_i64).unwrap(),
     );
@@ -154,7 +157,7 @@ impl DiagnosticItem {
 
         for related_info_v in related_info_values {
           related_information
-            .push(DiagnosticItem::from_json_value(related_info_v));
+            .push(DiagnosticItem::from_json_value(related_info_v)?);
         }
 
         Some(related_information)
@@ -162,7 +165,7 @@ impl DiagnosticItem {
       _ => None,
     };
 
-    Self {
+    Some(Self {
       message,
       message_chain,
       related_information,
@@ -175,7 +178,7 @@ impl DiagnosticItem {
       category,
       start_column,
       end_column,
-    }
+    })
   }
 }
 
diff --git a/cli/js/deno.ts b/cli/js/deno.ts
index 887f608260..f5f7741799 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 fc1c69a62d..1b7e234632 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 d1d5a73f21..63250473b1 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 0000000000..282c2b274b
--- /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 0c94308691..4d7822c907 100644
--- a/cli/js/lib.deno.ns.d.ts
+++ b/cli/js/lib.deno.ns.d.ts
@@ -1901,6 +1901,13 @@ declare namespace Deno {
     items: DiagnosticItem[];
   }
 
+  /** 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
diff --git a/cli/js/unit_tests.ts b/cli/js/unit_tests.ts
index ec4505c219..1c82374663 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";
diff --git a/cli/ops/errors.rs b/cli/ops/errors.rs
index 4dd4f1ca19..b5cc75f7ad 100644
--- a/cli/ops/errors.rs
+++ b/cli/ops/errors.rs
@@ -1,5 +1,6 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
 use super::dispatch_json::{Deserialize, JsonOp, Value};
+use crate::diagnostics::Diagnostic;
 use crate::fmt_errors::JSError;
 use crate::op_error::OpError;
 use crate::ops::json_op;
@@ -18,6 +19,10 @@ pub fn init(i: &mut Isolate, s: &State) {
     "format_error",
     s.core_op(json_op(s.stateful_op(op_format_error))),
   );
+  i.register_op(
+    "format_diagnostic",
+    s.core_op(json_op(s.stateful_op(op_format_diagnostic))),
+  );
 }
 
 #[derive(Deserialize)]
@@ -68,3 +73,15 @@ fn op_apply_source_map(
     "column": orig_column as u32,
   })))
 }
+
+fn op_format_diagnostic(
+  _state: &State,
+  args: Value,
+  _zero_copy: Option<ZeroCopyBuf>,
+) -> Result<JsonOp, OpError> {
+  if let Some(diagnostic) = Diagnostic::from_json_value(&args) {
+    Ok(JsonOp::Sync(json!(diagnostic.to_string())))
+  } else {
+    Err(OpError::type_error("bad diagnostic".to_string()))
+  }
+}