diff --git a/package-lock.json b/package-lock.json
index ae3b0d16ef..a468055873 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -4720,6 +4720,12 @@
           "resolved": "https://registry.npmjs.org/ms/-/ms-2.0.0.tgz",
           "integrity": "sha1-VgiurfwAvmwpAd9fmGF4jeDVl8g=",
           "dev": true
+        },
+        "source-map": {
+          "version": "0.5.7",
+          "resolved": "https://registry.npmjs.org/source-map/-/source-map-0.5.7.tgz",
+          "integrity": "sha1-igOdLRAh0i0eoUyA2OpGi6LvP8w=",
+          "dev": true
         }
       }
     },
@@ -4800,12 +4806,6 @@
       "integrity": "sha512-qnQ7gVMxGNxsiL4lEuJwe/To8UnK7fAnmbGEEH8RpLouuKbeEm0lhbQVFIrNSuB+G7tVrAlVsZgETT5nljf+Iw==",
       "dev": true
     },
-    "source-map": {
-      "version": "0.5.7",
-      "resolved": "https://registry.npmjs.org/source-map/-/source-map-0.5.7.tgz",
-      "integrity": "sha1-igOdLRAh0i0eoUyA2OpGi6LvP8w=",
-      "dev": true
-    },
     "source-map-resolve": {
       "version": "0.5.3",
       "resolved": "https://registry.npmjs.org/source-map-resolve/-/source-map-resolve-0.5.3.tgz",
diff --git a/src/chromium/crCoverage.ts b/src/chromium/crCoverage.ts
index aea6d16661..616ef1f885 100644
--- a/src/chromium/crCoverage.ts
+++ b/src/chromium/crCoverage.ts
@@ -18,8 +18,7 @@
 import { CRSession } from './crConnection';
 import { assert, helper, RegisteredListener } from '../helper';
 import { Protocol } from './protocol';
-
-import { EVALUATION_SCRIPT_URL } from './crExecutionContext';
+import * as js from '../javascript';
 import * as types from '../types';
 import { logError, InnerLogger } from '../logger';
 
@@ -126,7 +125,7 @@ class JSCoverage {
 
   async _onScriptParsed(event: Protocol.Debugger.scriptParsedPayload) {
     // Ignore playwright-injected scripts
-    if (event.url === EVALUATION_SCRIPT_URL)
+    if (js.isPlaywrightSourceUrl(event.url))
       return;
     this._scriptIds.add(event.scriptId);
     // Ignore other anonymous scripts unless the reportAnonymousScripts option is true.
diff --git a/src/chromium/crExecutionContext.ts b/src/chromium/crExecutionContext.ts
index 90cf79b174..9a32521864 100644
--- a/src/chromium/crExecutionContext.ts
+++ b/src/chromium/crExecutionContext.ts
@@ -21,9 +21,6 @@ import { valueFromRemoteObject, getExceptionMessage, releaseObject } from './crP
 import { Protocol } from './protocol';
 import * as js from '../javascript';
 
-export const EVALUATION_SCRIPT_URL = '__playwright_evaluation_script__';
-const SOURCE_URL_REGEX = /^[\040\t]*\/\/[@#] sourceURL=\s*(\S*?)\s*$/m;
-
 export class CRExecutionContext implements js.ExecutionContextDelegate {
   _client: CRSession;
   _contextId: number;
@@ -34,14 +31,11 @@ export class CRExecutionContext implements js.ExecutionContextDelegate {
   }
 
   async evaluate(context: js.ExecutionContext, returnByValue: boolean, pageFunction: Function | string, ...args: any[]): Promise<any> {
-    const suffix = `//# sourceURL=${EVALUATION_SCRIPT_URL}`;
-
     if (helper.isString(pageFunction)) {
       const contextId = this._contextId;
       const expression: string = pageFunction;
-      const expressionWithSourceUrl = SOURCE_URL_REGEX.test(expression) ? expression : expression + '\n' + suffix;
       const {exceptionDetails, result: remoteObject} = await this._client.send('Runtime.evaluate', {
-        expression: expressionWithSourceUrl,
+        expression: js.ensureSourceUrl(expression),
         contextId,
         returnByValue,
         awaitPromise: true,
@@ -79,7 +73,7 @@ export class CRExecutionContext implements js.ExecutionContextDelegate {
 
     try {
       const { exceptionDetails, result: remoteObject } = await this._client.send('Runtime.callFunctionOn', {
-        functionDeclaration: functionText + '\n' + suffix + '\n',
+        functionDeclaration: functionText,
         executionContextId: this._contextId,
         arguments: [
           ...values.map(value => ({ value })),
diff --git a/src/chromium/crPage.ts b/src/chromium/crPage.ts
index 31cd76b617..858c677e08 100644
--- a/src/chromium/crPage.ts
+++ b/src/chromium/crPage.ts
@@ -21,7 +21,7 @@ import * as frames from '../frames';
 import { helper, RegisteredListener, assert } from '../helper';
 import * as network from '../network';
 import { CRSession, CRConnection, CRSessionEvents } from './crConnection';
-import { EVALUATION_SCRIPT_URL, CRExecutionContext } from './crExecutionContext';
+import { CRExecutionContext } from './crExecutionContext';
 import { CRNetworkManager } from './crNetworkManager';
 import { Page, Worker, PageBinding } from '../page';
 import { Protocol } from './protocol';
@@ -418,7 +418,7 @@ class FrameSession {
       lifecycleEventsEnabled = this._client.send('Page.setLifecycleEventsEnabled', { enabled: true }),
       this._client.send('Runtime.enable', {}),
       this._client.send('Page.addScriptToEvaluateOnNewDocument', {
-        source: `//# sourceURL=${EVALUATION_SCRIPT_URL}`,
+        source: js.generateSourceUrl(),
         worldName: UTILITY_WORLD_NAME,
       }),
       this._networkManager.initialize(),
diff --git a/src/firefox/ffExecutionContext.ts b/src/firefox/ffExecutionContext.ts
index 768636b35e..a432ef5504 100644
--- a/src/firefox/ffExecutionContext.ts
+++ b/src/firefox/ffExecutionContext.ts
@@ -32,7 +32,7 @@ export class FFExecutionContext implements js.ExecutionContextDelegate {
   async evaluate(context: js.ExecutionContext, returnByValue: boolean, pageFunction: Function | string, ...args: any[]): Promise<any> {
     if (helper.isString(pageFunction)) {
       const payload = await this._session.send('Runtime.evaluate', {
-        expression: pageFunction.trim(),
+        expression: js.ensureSourceUrl(pageFunction),
         returnByValue,
         executionContextId: this._executionContextId,
       }).catch(rewriteError);
@@ -117,7 +117,7 @@ export class FFExecutionContext implements js.ExecutionContextDelegate {
     const simpleValue = await this._session.send('Runtime.callFunction', {
       executionContextId: this._executionContextId,
       returnByValue: true,
-      functionDeclaration: ((e: any) => e).toString(),
+      functionDeclaration: ((e: any) => e).toString() + js.generateSourceUrl(),
       args: [this._toCallArgument(payload)],
     });
     return deserializeValue(simpleValue.result!);
diff --git a/src/helper.ts b/src/helper.ts
index 6c82e6cd01..3a117c96f0 100644
--- a/src/helper.ts
+++ b/src/helper.ts
@@ -22,6 +22,9 @@ import * as util from 'util';
 import { TimeoutError } from './errors';
 import * as types from './types';
 
+// NOTE: update this to point to playwright/lib when moving this file.
+const PLAYWRIGHT_LIB_PATH = __dirname;
+
 export type RegisteredListener = {
   emitter: EventEmitter;
   eventName: (string | symbol);
@@ -370,6 +373,38 @@ export function logPolitely(toBeLogged: string) {
     console.log(toBeLogged);  // eslint-disable-line no-console
 }
 
+export function getCallerFilePath(ignorePrefix = PLAYWRIGHT_LIB_PATH): string | null {
+  const error = new Error();
+  const stackFrames = (error.stack || '').split('\n').slice(1);
+  // Find first stackframe that doesn't point to ignorePrefix.
+  for (let frame of stackFrames) {
+    frame = frame.trim();
+    if (!frame.startsWith('at '))
+      return null;
+    if (frame.endsWith(')')) {
+      const from = frame.indexOf('(');
+      frame = frame.substring(from + 1, frame.length - 1);
+    } else {
+      frame = frame.substring('at '.length);
+    }
+    const match = frame.match(/^(?:async )?(.*):(\d+):(\d+)$/);
+    if (!match)
+      return null;
+    const filePath = match[1];
+    if (filePath.startsWith(ignorePrefix))
+      continue;
+    return filePath;
+  }
+  return null;
+}
+
+let debugMode: boolean | undefined;
+export function isDebugMode(): boolean {
+  if (debugMode === undefined)
+    debugMode = !!getFromENV('PLAYWRIGHT_DEBUG_UI');
+  return debugMode;
+}
+
 const escapeGlobChars = new Set(['/', '$', '^', '+', '.', '(', ')', '=', '!', '|']);
 
 export const helper = Helper;
diff --git a/src/javascript.ts b/src/javascript.ts
index 45fcf076c1..09f3a92d74 100644
--- a/src/javascript.ts
+++ b/src/javascript.ts
@@ -16,7 +16,9 @@
 
 import * as types from './types';
 import * as dom from './dom';
-import { helper } from './helper';
+import * as fs from 'fs';
+import * as util from 'util';
+import { helper, getCallerFilePath, isDebugMode } from './helper';
 import { InnerLogger } from './logger';
 
 export interface ExecutionContextDelegate {
@@ -125,7 +127,8 @@ export async function prepareFunctionCall<T>(
   args: any[],
   toCallArgumentIfNeeded: (value: any) => { handle?: T, value?: any }): Promise<{ functionText: string, values: any[], handles: T[], dispose: () => void }> {
 
-  let functionText = pageFunction.toString();
+  const originalText = pageFunction.toString();
+  let functionText = originalText;
   try {
     new Function('(' + functionText + ')');
   } catch (e1) {
@@ -199,10 +202,13 @@ export async function prepareFunctionCall<T>(
   if (error)
     throw new Error(error);
 
-  if (!guids.length)
+  if (!guids.length) {
+    const sourceMapUrl = await generateSourceMapUrl(originalText, { line: 0, column: 0 });
+    functionText += sourceMapUrl;
     return { functionText, values: args, handles: [], dispose: () => {} };
+  }
 
-  functionText = `(...__playwright__args__) => {
+  const wrappedFunctionText = `(...__playwright__args__) => {
     return (${functionText})(...(() => {
       const args = __playwright__args__;
       __playwright__args__ = undefined;
@@ -226,6 +232,8 @@ export async function prepareFunctionCall<T>(
       return result;
     })());
   }`;
+  const compiledPosition = findPosition(wrappedFunctionText, wrappedFunctionText.indexOf(functionText));
+  functionText = wrappedFunctionText;
 
   const resolved = await Promise.all(handles);
   const resultHandles: T[] = [];
@@ -242,5 +250,120 @@ export async function prepareFunctionCall<T>(
   const dispose = () => {
     toDispose.map(handlePromise => handlePromise.then(handle => handle.dispose()));
   };
+
+  const sourceMapUrl = await generateSourceMapUrl(originalText, compiledPosition);
+  functionText += sourceMapUrl;
   return { functionText, values: [ args.length, ...args, guids.length, ...guids ], handles: resultHandles, dispose };
 }
+
+let sourceUrlCounter = 0;
+const playwrightSourceUrlPrefix = '__playwright_evaluation_script__';
+const sourceUrlRegex = /^[\040\t]*\/\/[@#] sourceURL=\s*(\S*?)\s*$/m;
+export function generateSourceUrl(): string {
+  return `\n//# sourceURL=${playwrightSourceUrlPrefix}${sourceUrlCounter++}\n`;
+}
+
+export function isPlaywrightSourceUrl(s: string): boolean {
+  return s.startsWith(playwrightSourceUrlPrefix);
+}
+
+export function ensureSourceUrl(expression: string): string {
+  return sourceUrlRegex.test(expression) ? expression : expression + generateSourceUrl();
+}
+
+type Position = {
+  line: number;
+  column: number;
+};
+
+async function generateSourceMapUrl(functionText: string, compiledPosition: Position): Promise<string> {
+  if (!isDebugMode())
+    return generateSourceUrl();
+  const filePath = getCallerFilePath();
+  if (!filePath)
+    return generateSourceUrl();
+  try {
+    const source = await util.promisify(fs.readFile)(filePath, 'utf8');
+    const index = source.indexOf(functionText);
+    if (index === -1)
+      return generateSourceUrl();
+    const sourcePosition = findPosition(source, index);
+    const delta = findPosition(functionText, functionText.length);
+    const sourceMap = generateSourceMap(filePath, sourcePosition, compiledPosition, delta);
+    return `\n//# sourceMappingURL=data:application/json;base64,${Buffer.from(sourceMap).toString('base64')}\n`;
+  } catch (e) {
+    return generateSourceUrl();
+  }
+}
+
+const VLQ_BASE_SHIFT = 5;
+const VLQ_BASE = 1 << VLQ_BASE_SHIFT;
+const VLQ_BASE_MASK = VLQ_BASE - 1;
+const VLQ_CONTINUATION_BIT = VLQ_BASE;
+const BASE64_DIGITS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';
+
+function base64VLQ(value: number): string {
+  if (value < 0)
+    value = ((-value) << 1) | 1;
+  else
+    value <<= 1;
+  let result = '';
+  do {
+    let digit = value & VLQ_BASE_MASK;
+    value >>>= VLQ_BASE_SHIFT;
+    if (value > 0)
+      digit |= VLQ_CONTINUATION_BIT;
+    result += BASE64_DIGITS[digit];
+  } while (value > 0);
+  return result;
+}
+
+function generateSourceMap(filePath: string, sourcePosition: Position, compiledPosition: Position, delta: Position): any {
+  const mappings = [];
+  let lastCompiled = { line: 0, column: 0 };
+  let lastSource = { line: 0, column: 0 };
+  for (let line = 0; line < delta.line; line++) {
+    // We need at least a mapping per line. This will yield an execution line at the start of each line.
+    // Note: for more granular mapping, we can do word-by-word.
+    const source = advancePosition(sourcePosition, { line, column: 0 });
+    const compiled = advancePosition(compiledPosition, { line, column: 0 });
+    while (lastCompiled.line < compiled.line) {
+      mappings.push(';');
+      lastCompiled.line++;
+      lastCompiled.column = 0;
+    }
+    mappings.push(base64VLQ(compiled.column - lastCompiled.column));
+    mappings.push(base64VLQ(0)); // Source index.
+    mappings.push(base64VLQ(source.line - lastSource.line));
+    mappings.push(base64VLQ(source.column - lastSource.column));
+    lastCompiled = compiled;
+    lastSource = source;
+  }
+  return JSON.stringify({
+    version: 3,
+    sources: ['file://' + filePath],
+    names: [],
+    mappings: mappings.join(''),
+  });
+}
+
+function findPosition(source: string, offset: number): Position {
+  const result: Position = { line: 0, column: 0 };
+  let index = 0;
+  while (true) {
+    const newline = source.indexOf('\n', index);
+    if (newline === -1 || newline >= offset)
+      break;
+    result.line++;
+    index = newline + 1;
+  }
+  result.column = offset - index;
+  return result;
+}
+
+function advancePosition(position: Position, delta: Position) {
+  return {
+    line: position.line + delta.line,
+    column: delta.column + (delta.line ? 0 : position.column),
+  };
+}
diff --git a/src/webkit/wkExecutionContext.ts b/src/webkit/wkExecutionContext.ts
index 0af7d2c37f..06c5548fe6 100644
--- a/src/webkit/wkExecutionContext.ts
+++ b/src/webkit/wkExecutionContext.ts
@@ -21,9 +21,6 @@ import { valueFromRemoteObject, releaseObject } from './wkProtocolHelper';
 import { Protocol } from './protocol';
 import * as js from '../javascript';
 
-export const EVALUATION_SCRIPT_URL = '__playwright_evaluation_script__';
-const SOURCE_URL_REGEX = /^[\040\t]*\/\/[@#] sourceURL=\s*(\S*?)\s*$/m;
-
 type MaybeCallArgument = Protocol.Runtime.CallArgument | { unserializable: any };
 
 export class WKExecutionContext implements js.ExecutionContextDelegate {
@@ -75,9 +72,8 @@ export class WKExecutionContext implements js.ExecutionContextDelegate {
     if (helper.isString(pageFunction)) {
       const contextId = this._contextId;
       const expression: string = pageFunction;
-      const expressionWithSourceUrl = SOURCE_URL_REGEX.test(expression) ? expression : expression + '\n' + suffix;
       return await this._session.send('Runtime.evaluate', {
-        expression: expressionWithSourceUrl,
+        expression: js.ensureSourceUrl(expression),
         contextId,
         returnByValue: false,
         emulateUserGesture: true
@@ -105,7 +101,7 @@ export class WKExecutionContext implements js.ExecutionContextDelegate {
       const callParams = this._serializeFunctionAndArguments(functionText, values, handles);
       const thisObjectId = await this._contextGlobalObjectId();
       return await this._session.send('Runtime.callFunctionOn', {
-        functionDeclaration: callParams.functionText + '\n' + suffix + '\n',
+        functionDeclaration: callParams.functionText,
         objectId: thisObjectId,
         arguments: callParams.callArguments,
         returnByValue: false,
@@ -170,7 +166,7 @@ export class WKExecutionContext implements js.ExecutionContextDelegate {
     try {
       const serializeResponse = await this._session.send('Runtime.callFunctionOn', {
         // Serialize object using standard JSON implementation to correctly pass 'undefined'.
-        functionDeclaration: 'function(){return this}\n' + suffix + '\n',
+        functionDeclaration: 'function(){return this}\n' + js.generateSourceUrl(),
         objectId: objectId,
         returnByValue: true
       });
@@ -231,7 +227,6 @@ export class WKExecutionContext implements js.ExecutionContextDelegate {
   }
 }
 
-const suffix = `//# sourceURL=${EVALUATION_SCRIPT_URL}`;
 const contextDestroyedResult = {
   wasThrown: true,
   result: {