errors: print original exception context

When --enable-source-maps is set, the error context displayed
above the stack trace now shows original source rather than
transpiled.

PR-URL: https://github.com/nodejs/node/pull/33491
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

2 files changed, 76 insertions, 8 deletions

diff --git a/lib/internal/source_map/prepare_stack_trace.js b/lib/internal/source_map/prepare_stack_trace.js
index 037a8dc53e0..df0d79a204b 100644
--- a/lib/internal/source_map/prepare_stack_trace.js
+++ b/lib/internal/source_map/prepare_stack_trace.js
@@ -5,6 +5,8 @@ const {
 } = primordials;
 
 const debug = require('internal/util/debuglog').debuglog('source_map');
+const { getStringWidth } = require('internal/util/inspect');
+const { readFileSync } = require('fs');
 const { findSourceMap } = require('internal/source_map/source_map_cache');
 const {
   kNoOverride,
@@ -34,7 +36,17 @@ const prepareStackTrace = (globalThis, error, trace) => {
   if (trace.length === 0) {
     return errorString;
   }
+
+  let errorSource = '';
+  let firstSource;
+  let firstLine;
+  let firstColumn;
   const preparedTrace = trace.map((t, i) => {
+    if (i === 0) {
+      firstLine = t.getLineNumber();
+      firstColumn = t.getColumnNumber();
+      firstSource = t.getFileName();
+    }
     let str = i !== 0 ? '\n    at ' : '';
     str = `${str}${t}`;
     try {
@@ -49,8 +61,18 @@ const prepareStackTrace = (globalThis, error, trace) => {
         } = sm.findEntry(t.getLineNumber() - 1, t.getColumnNumber() - 1);
         if (originalSource && originalLine !== undefined &&
             originalColumn !== undefined) {
-          str +=
-`\n        -> ${originalSource.replace('file://', '')}:${originalLine + 1}:${originalColumn + 1}`;
+          const originalSourceNoScheme = originalSource
+            .replace(/^file:\/\//, '');
+          if (i === 0) {
+            firstLine = originalLine + 1;
+            firstColumn = originalColumn + 1;
+            firstSource = originalSourceNoScheme;
+            // Show error in original source context to help user pinpoint it:
+            errorSource = getErrorSource(firstSource, firstLine, firstColumn);
+          }
+          // Show both original and transpiled stack trace information:
+          str += `\n        -> ${originalSourceNoScheme}:${originalLine + 1}:` +
+            `${originalColumn + 1}`;
         }
       }
     } catch (err) {
@@ -58,9 +80,38 @@ const prepareStackTrace = (globalThis, error, trace) => {
     }
     return str;
   });
-  return `${errorString}\n    at ${preparedTrace.join('')}`;
+  return `${errorSource}${errorString}\n    at ${preparedTrace.join('')}`;
 };
 
+// Places a snippet of code from where the exception was originally thrown
+// above the stack trace. This logic is modeled after GetErrorSource in
+// node_errors.cc.
+function getErrorSource(firstSource, firstLine, firstColumn) {
+  let exceptionLine = '';
+  let source;
+  try {
+    source = readFileSync(firstSource, 'utf8');
+  } catch (err) {
+    debug(err);
+    return exceptionLine;
+  }
+  const lines = source.split(/\r?\n/, firstLine);
+  const line = lines[firstLine - 1];
+  if (!line) return exceptionLine;
+
+  // Display ^ in appropriate position, regardless of whether tabs or
+  // spaces are used:
+  let prefix = '';
+  for (const character of line.slice(0, firstColumn)) {
+    prefix += (character === '\t') ? '\t' :
+      ' '.repeat(getStringWidth(character));
+  }
+  prefix = prefix.slice(0, -1); // The last character is the '^'.
+
+  exceptionLine = `${firstSource}:${firstLine}\n${line}\n${prefix}^\n\n`;
+  return exceptionLine;
+}
+
 module.exports = {
   prepareStackTrace,
 };
diff --git a/lib/internal/source_map/source_map_cache.js b/lib/internal/source_map/source_map_cache.js
index 06b1a2a5f52..04926acfc1b 100644
--- a/lib/internal/source_map/source_map_cache.js
+++ b/lib/internal/source_map/source_map_cache.js
@@ -39,12 +39,28 @@ const { fileURLToPath, URL } = require('url');
 let Module;
 let SourceMap;
 
-let experimentalSourceMaps;
-function maybeCacheSourceMap(filename, content, cjsModuleInstance) {
-  if (experimentalSourceMaps === undefined) {
-    experimentalSourceMaps = getOptionValue('--enable-source-maps');
+let sourceMapsEnabled;
+function getSourceMapsEnabled() {
+  if (sourceMapsEnabled === undefined) {
+    sourceMapsEnabled = getOptionValue('--enable-source-maps');
+    if (sourceMapsEnabled) {
+      const {
+        enableSourceMaps,
+        setPrepareStackTraceCallback
+      } = internalBinding('errors');
+      const {
+        prepareStackTrace
+      } = require('internal/source_map/prepare_stack_trace');
+      setPrepareStackTraceCallback(prepareStackTrace);
+      enableSourceMaps();
+    }
   }
-  if (!(process.env.NODE_V8_COVERAGE || experimentalSourceMaps)) return;
+  return sourceMapsEnabled;
+}
+
+function maybeCacheSourceMap(filename, content, cjsModuleInstance) {
+  const sourceMapsEnabled = getSourceMapsEnabled();
+  if (!(process.env.NODE_V8_COVERAGE || sourceMapsEnabled)) return;
   let basePath;
   try {
     filename = normalizeReferrerURL(filename);
@@ -248,6 +264,7 @@ function findSourceMap(uri, error) {
 
 module.exports = {
   findSourceMap,
+  getSourceMapsEnabled,
   maybeCacheSourceMap,
   rekeySourceMap,
   sourceMapCacheToObject,