diff --git a/.gitignore b/.gitignore
index 57082f26a3..2efb8995ec 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,4 +6,5 @@ out/
 raw/
 .history
 *.backup
-.vscode
\ No newline at end of file
+.vscode
+.idea
diff --git a/NOTICE b/NOTICE
index 8e47e0290f..1a15f844c7 100644
--- a/NOTICE
+++ b/NOTICE
@@ -30,6 +30,7 @@ under the licensing terms detailed in LICENSE:
 * Gabor Greif <ggreif@gmail.com>
 * Martin Fredriksson <martin.fredriksson@vikinganalytics.se>
 * forcepusher <bionitsoup@gmail.com>
+* Piotr Oleś <piotrek.oles@gmail.com>
 
 Portions of this software are derived from third-party works licensed under
 the following terms:
diff --git a/cli/asc.d.ts b/cli/asc.d.ts
index 7957ac7c2f..ecf3c9589c 100644
--- a/cli/asc.d.ts
+++ b/cli/asc.d.ts
@@ -55,6 +55,38 @@ export interface MemoryStream extends OutputStream {
   toString(): string;
 }
 
+/** A subset of Source interface from assemblyscript package */
+export interface Source {
+  /** Normalized path with file extension. */
+  normalizedPath: string;
+}
+
+/** A subset of Range interface from assemblyscript package */
+export interface Range {
+  /** Range start char index in a source */
+  start: number;
+  /** Range end char index in a source */
+  end: number;
+  source: Source;
+}
+
+/** A subset of DiagnosticMessage interface from assemblyscript package */
+export interface DiagnosticMessage {
+  /** Message code. */
+  code: number;
+  /** Message category. */
+  category: number;
+  /** Message text. */
+  message: string;
+  /** Respective source range, if any. */
+  range: Range | null;
+  /** Related range, if any. */
+  relatedRange: Range | null;
+}
+
+/** A function that handles diagnostic */
+type DiagnosticReporter = (diagnostic: DiagnosticMessage) => void;
+
 /** Compiler options. */
 export interface CompilerOptions {
   /** Prints just the compiler's version and exits. */
@@ -157,6 +189,8 @@ export interface APIOptions {
   writeFile?: (filename: string, contents: Uint8Array, baseDir: string) => void;
   /** Lists all files within a directory. */
   listFiles?: (dirname: string, baseDir: string) => string[] | null;
+  /** Handles diagnostic messages. */
+  reportDiagnostic?: DiagnosticReporter;
 }
 
 /** Convenience function that parses and compiles source strings directly. */
@@ -176,7 +210,7 @@ export function main(argv: string[], options: APIOptions, callback?: (err: Error
 export function main(argv: string[], callback?: (err: Error | null) => number): number;
 
 /** Checks diagnostics emitted so far for errors. */
-export function checkDiagnostics(emitter: Record<string,unknown>, stderr?: OutputStream): boolean;
+export function checkDiagnostics(emitter: Record<string,unknown>, stderr?: OutputStream, reportDiagnostic?: DiagnosticReporter): boolean;
 
 /** An object of stats for the current task. */
 export interface Stats {
diff --git a/cli/asc.js b/cli/asc.js
index e288bda790..a3c31ec163 100644
--- a/cli/asc.js
+++ b/cli/asc.js
@@ -129,6 +129,20 @@ function loadAssemblyScriptWasm(binaryPath) {
   return exports;
 }
 
+/**
+ * Wraps object in WASM environment, returns untouched otherwise.
+ * @template T
+ * @param {number | T} ptrOrObj Pointer in WASM environment, object otherwise
+ * @param {typeof T} type Object type that provides wrap method in WASM environment
+ * @returns {T | null}
+ */
+function __wrap(ptrOrObj, type) {
+  if (typeof ptrOrObj === "number") {
+    return ptrOrObj === 0 ? null : type.wrap(ptrOrObj);
+  }
+  return ptrOrObj;
+}
+
 var assemblyscript, __newString, __getString, __pin, __unpin, __collect;
 
 function loadAssemblyScript() {
@@ -702,7 +716,7 @@ exports.main = function main(argv, options, callback) {
         });
       }
     }
-    var numErrors = checkDiagnostics(program, stderr);
+    var numErrors = checkDiagnostics(program, stderr, options.reportDiagnostic);
     if (numErrors) {
       const err = Error(numErrors + " parse error(s)");
       err.stack = err.message; // omit stack
@@ -816,7 +830,7 @@ exports.main = function main(argv, options, callback) {
       };
     }
   });
-  var numErrors = checkDiagnostics(program, stderr);
+  var numErrors = checkDiagnostics(program, stderr, options.reportDiagnostic);
   if (numErrors) {
     if (module) module.dispose();
     const err = Error(numErrors + " compile error(s)");
@@ -1155,7 +1169,7 @@ function getAsconfig(file, baseDir, readFile) {
 exports.getAsconfig = getAsconfig;
 
 /** Checks diagnostics emitted so far for errors. */
-function checkDiagnostics(program, stderr) {
+function checkDiagnostics(program, stderr, reportDiagnostic) {
   var numErrors = 0;
   do {
     let diagnosticPtr = assemblyscript.nextDiagnostic(program);
@@ -1167,6 +1181,33 @@ function checkDiagnostics(program, stderr) {
         EOL + EOL
       );
     }
+    if (reportDiagnostic) {
+      const diagnostic = __wrap(diagnosticPtr, assemblyscript.DiagnosticMessage);
+      const range = __wrap(diagnostic.range, assemblyscript.Range);
+      const relatedRange = __wrap(diagnostic.relatedRange, assemblyscript.Range);
+      const rangeSource = range ? __wrap(range.source, assemblyscript.Source) : null;
+      const relatedRangeSource = relatedRange ? __wrap(relatedRange.source, assemblyscript.Source) : null;
+
+      reportDiagnostic({
+        message: __getString(diagnostic.message),
+        code: diagnostic.code,
+        category: diagnostic.category,
+        range: range ? {
+          start: range.start,
+          end: range.end,
+          source: rangeSource ? {
+            normalizedPath: __getString(rangeSource.normalizedPath)
+          } : null,
+        } : null,
+        relatedRange: relatedRange ? {
+          start: relatedRange.start,
+          end: relatedRange.end,
+          source: relatedRangeSource ? {
+            normalizedPath: __getString(relatedRangeSource.normalizedPath)
+          } : null
+        } : null
+      });
+    }
     if (assemblyscript.isError(diagnosticPtr)) ++numErrors;
     __unpin(diagnosticPtr);
   } while (true);