From a7bf2bea15f10ff39348d470ca19a99d02415b3d Mon Sep 17 00:00:00 2001
From: hemanth <hemanth.hm@gmail.com>
Date: Sat, 18 Jul 2026 13:54:52 -0700
Subject: [PATCH] repl: add syntax highlighting, auto-indentation, and
 signature hints

Signed-off-by: hemanth <hemanth.hm@gmail.com>
---
 doc/api/repl.md                               |  54 ++-
 lib/internal/repl/utils.js                    | 457 ++++++++++++++++++
 lib/repl.js                                   |  30 ++
 test/parallel/test-repl-auto-indent.js        | 249 ++++++++++
 .../parallel/test-repl-syntax-highlighting.js | 293 +++++++++++
 5 files changed, 1082 insertions(+), 1 deletion(-)
 create mode 100644 test/parallel/test-repl-auto-indent.js
 create mode 100644 test/parallel/test-repl-syntax-highlighting.js

diff --git a/doc/api/repl.md b/doc/api/repl.md
index c384ebf1391303..6475de353d7583 100644
--- a/doc/api/repl.md
+++ b/doc/api/repl.md
@@ -27,7 +27,8 @@ result. Input and output may be from `stdin` and `stdout`, respectively, or may
 be connected to any Node.js [stream][].
 
 Instances of [`repl.REPLServer`][] support automatic completion of inputs,
-completion preview, simplistic Emacs-style line editing, multi-line inputs,
+completion preview, inline syntax highlighting, automatic indentation,
+function signature hints, simplistic Emacs-style line editing, multi-line inputs,
 [ZSH][]-like reverse-i-search, [ZSH][]-like substring-based history search,
 ANSI-styled output, saving and restoring current REPL session state, error
 recovery, and customizable evaluation functions. Terminals that do not support
@@ -232,6 +233,52 @@ undefined
 undefined
 ```
 
+### Syntax highlighting
+
+<!-- YAML
+added: REPLACEME
+-->
+
+When `useColors` is `true`, the REPL highlights
+JavaScript input using the following color scheme:
+
+* **Keywords** (`const`, `let`, `function`, `if`, `return`, etc.): Magenta
+* **Strings** (single-quoted, double-quoted, and template literals): Green
+* **Numbers**: Yellow
+* **Boolean literals** (`true`, `false`), `null`, `undefined`, `NaN`,
+  `Infinity`: Yellow
+* **Regular expressions**: Red
+* **Comments** (line and block): Gray
+
+Syntax highlighting is applied only to the display; the internal `line`
+property remains plain text. Highlighting can be disabled by passing
+`syntaxHighlighting: false` to [`repl.start()`][].
+
+### Auto-indentation
+
+<!-- YAML
+added: REPLACEME
+-->
+
+When entering
+multi-line input (e.g., a function body or an object literal), the REPL
+automatically indents continuation lines based on the current nesting
+depth of braces (`{}`), brackets (`[]`), and parentheses (`()`). The REPL
+uses 2-space indentation following the Node.js coding convention.
+
+### Function signature hints
+
+<!-- YAML
+added: REPLACEME
+-->
+
+When the user
+types a function name followed by `(`, the REPL displays the function's
+parameter list as a dimmed hint below the input line. This uses the V8
+inspector protocol to safely extract function signatures without side
+effects. Signature hints require the `preview` option to be enabled and
+the inspector to be available.
+
 ### Reverse-i-search
 
 <!-- YAML
@@ -747,6 +794,11 @@ changes:
     previews or not. **Default:** `true` with the default eval function and
     `false` in case a custom eval function is used. If `terminal` is falsy, then
     there are no previews and the value of `preview` has no effect.
+  * `syntaxHighlighting` {boolean} If `true`, enables inline syntax
+    highlighting of REPL input using ANSI color codes. Keywords are displayed
+    in magenta, strings in green, numbers in yellow, regular expressions in
+    red, and comments in gray. Requires `useColors` to be `true` and `terminal` to be `true`.
+    **Default:** `true`.
   * `handleError` {Function} This function customizes error handling in the REPL.
     It receives the thrown exception as its first argument and must return one
     of the following values synchronously:
diff --git a/lib/internal/repl/utils.js b/lib/internal/repl/utils.js
index 750867d6ba4f8a..5b34e2520d6ce0 100644
--- a/lib/internal/repl/utils.js
+++ b/lib/internal/repl/utils.js
@@ -3,7 +3,9 @@
 const {
   ArrayPrototypeFilter,
   ArrayPrototypeIncludes,
+  ArrayPrototypeJoin,
   ArrayPrototypeMap,
+  ArrayPrototypePush,
   Boolean,
   FunctionPrototypeBind,
   MathMin,
@@ -11,6 +13,7 @@ const {
   RegExpPrototypeExec,
   SafeSet,
   SafeStringIterator,
+  StringPrototypeCharCodeAt,
   StringPrototypeIndexOf,
   StringPrototypeLastIndexOf,
   StringPrototypeReplaceAll,
@@ -861,6 +864,456 @@ function setReplBuiltinLibs(value) {
   _builtinLibs = value;
 }
 
+
+// Syntax highlighting uses util.styleText to dogfood Node.js core's
+// own coloring API. The color names align with util.inspect.styles
+// (e.g. 'magenta' for keywords, 'green' for strings, 'yellow' for numbers).
+const { styleText } = require('util');
+const kStyleOpts = { __proto__: null, validateStream: false };
+
+/**
+ * Wraps text with an ANSI color via util.styleText.
+ * Uses the fast path (validateStream: false) to skip stream checks.
+ * @param {string} colorName The color name (e.g. 'magenta', 'green').
+ * @param {string} text The text to colorize.
+ * @returns {string} The colorized text.
+ */
+function style(colorName, text) {
+  return styleText(colorName, text, kStyleOpts);
+}
+
+/**
+ * Applies ANSI syntax highlighting to a JavaScript code string.
+ * Uses the Acorn tokenizer to identify token types and wrap them
+ * in appropriate ANSI color escape sequences.
+ * @param {string} code The JavaScript code string to highlight.
+ * @param {boolean} useColors Whether to apply color codes.
+ * @returns {string} The highlighted code string, or the original if
+ *   useColors is false or tokenization fails.
+ */
+function syntaxHighlight(code, useColors) {
+  if (!useColors || code === '') {
+    return code;
+  }
+
+  const {
+    tokTypes: tt,
+    tokenizer: createTokenizer,
+  } = require('internal/deps/acorn/acorn/dist/acorn');
+
+  const parts = [];
+  let lastEnd = 0;
+
+  try {
+    const tokenizer = createTokenizer(code, {
+      ecmaVersion: 'latest',
+      allowAwaitOutsideFunction: true,
+      allowImportExportEverywhere: true,
+      allowReturnOutsideFunction: true,
+      allowSuperOutsideMethod: true,
+    });
+
+    // Collect comments via the onComment callback since the tokenizer
+    // iterator does not yield comment tokens.
+    const comments = [];
+    tokenizer.options.onComment = (block, text, start, end) => {
+      ArrayPrototypePush(comments, { start, end });
+    };
+
+    for (const token of tokenizer) {
+      // First, handle any comments that occur before this token.
+      while (comments.length > 0 && comments[0].start < token.start) {
+        const comment = comments[0];
+        // Add any text between last position and comment start.
+        if (comment.start > lastEnd) {
+          ArrayPrototypePush(
+            parts, StringPrototypeSlice(code, lastEnd, comment.start));
+        }
+        ArrayPrototypePush(parts,
+                           style('gray',
+                                 StringPrototypeSlice(code, comment.start,
+                                                      comment.end)));
+        lastEnd = comment.end;
+        comments.shift();
+      }
+
+      // Add any unhighlighted text between the last token and this one.
+      if (token.start > lastEnd) {
+        ArrayPrototypePush(
+          parts, StringPrototypeSlice(code, lastEnd, token.start));
+      }
+
+      const tokenText = StringPrototypeSlice(code, token.start, token.end);
+      let colorName;
+
+      if (token.type.keyword) {
+        // Keywords: const, let, var, function, if, else, etc.
+        // Also includes true, false, null which have keyword set.
+        const kw = token.type.keyword;
+        if (kw === 'true' || kw === 'false' || kw === 'null') {
+          colorName = 'yellow';
+        } else {
+          colorName = 'magenta';
+        }
+      } else if (token.type === tt.num) {
+        colorName = 'yellow';
+      } else if (token.type === tt.string) {
+        colorName = 'green';
+      } else if (token.type === tt.template || token.type === tt.backQuote ||
+                 token.type === tt.dollarBraceL ||
+                 token.type === tt.invalidTemplate) {
+        colorName = 'green';
+      } else if (token.type === tt.regexp) {
+        colorName = 'red';
+      } else if (token.type === tt.name &&
+                 (tokenText === 'undefined' || tokenText === 'NaN' ||
+                  tokenText === 'Infinity')) {
+        colorName = 'yellow';
+      } else if (token.type === tt.name &&
+                 (tokenText === 'let' || tokenText === 'const' ||
+                  tokenText === 'async' || tokenText === 'await' ||
+                  tokenText === 'class' || tokenText === 'yield' ||
+                  tokenText === 'of' || tokenText === 'static' ||
+                  tokenText === 'get' || tokenText === 'set')) {
+        colorName = 'magenta';
+      }
+
+      if (colorName) {
+        ArrayPrototypePush(parts, style(colorName, tokenText));
+      } else {
+        ArrayPrototypePush(parts, tokenText);
+      }
+
+      lastEnd = token.end;
+    }
+
+    // Handle any trailing comments.
+    while (comments.length > 0) {
+      const comment = comments[0];
+      if (comment.start > lastEnd) {
+        ArrayPrototypePush(
+          parts, StringPrototypeSlice(code, lastEnd, comment.start));
+      }
+      ArrayPrototypePush(parts,
+                         style('gray',
+                               StringPrototypeSlice(code, comment.start,
+                                                    comment.end)));
+      lastEnd = comment.end;
+      comments.shift();
+    }
+
+    // Append any remaining code after the last token.
+    if (lastEnd < code.length) {
+      ArrayPrototypePush(parts, StringPrototypeSlice(code, lastEnd));
+    }
+  } catch {
+    // If tokenization fails (e.g., due to incomplete input in REPL),
+    // fall back to returning the code without highlighting.
+    return code;
+  }
+
+  return ArrayPrototypeJoin(parts, '');
+}
+
+/**
+ * Hooks into the REPL's _writeToOutput to apply syntax highlighting
+ * to the current input line during _refreshLine. This ensures
+ * the user sees highlighted code without affecting the internal
+ * `this.line` property (which must remain plain text for correct
+ * cursor positioning and evaluation).
+ * @param {REPLServer} repl The REPL instance.
+ * @param {boolean} active Whether syntax highlighting should be enabled.
+ */
+function setupSyntaxHighlighting(repl, active) {
+  if (process.env.TERM === 'dumb' || !active || !repl.useColors) {
+    return;
+  }
+
+  const originalRefreshLine =
+    FunctionPrototypeBind(repl._refreshLine, repl);
+
+  let isRefreshing = false;
+
+  // Override _writeToOutput to apply highlighting during refresh.
+  const originalWriteToOutput =
+    FunctionPrototypeBind(repl._writeToOutput, repl);
+  repl._writeToOutput = (stringToWrite) => {
+    if (isRefreshing && stringToWrite.length > 0) {
+      const prompt = repl.getPrompt() || repl._prompt || '';
+      // Only highlight when the string starts with the prompt (i.e.,
+      // this is the main line being written during _refreshLine).
+      if (StringPrototypeStartsWith(stringToWrite, prompt) &&
+          stringToWrite !== prompt) {
+        const code = StringPrototypeSlice(stringToWrite, prompt.length);
+        const highlighted = syntaxHighlight(code, true);
+        return originalWriteToOutput(prompt + highlighted);
+      }
+    }
+    return originalWriteToOutput(stringToWrite);
+  };
+
+  // Wrap _refreshLine to set the isRefreshing flag.
+  repl._refreshLine = () => {
+    isRefreshing = true;
+    try {
+      originalRefreshLine();
+    } finally {
+      isRefreshing = false;
+    }
+  };
+}
+
+/**
+ * Sets up automatic indentation for multiline REPL input.
+ * After a newline is inserted (due to Recoverable error), this
+ * calculates the appropriate indentation level based on open
+ * braces/brackets/parentheses and inserts the correct number of spaces.
+ * @param {REPLServer} repl The REPL instance.
+ */
+function setupAutoIndent(repl) {
+  if (!repl.terminal) {
+    return;
+  }
+
+  const kIndent = '  '; // 2-space indentation (Node.js convention)
+
+  /**
+   * Calculates the net depth change from unmatched braces, brackets,
+   * and parentheses in a given code string.
+   * @returns {number} The net depth change.
+   */
+  function calculateDepth(code) {
+    let depth = 0;
+    let inString = false;
+    let stringChar = '';
+    let inTemplate = false;
+    let inLineComment = false;
+    let inBlockComment = false;
+    let escaped = false;
+
+    for (let i = 0; i < code.length; i++) {
+      const ch = StringPrototypeCharCodeAt(code, i);
+
+      if (escaped) {
+        escaped = false;
+        continue;
+      }
+
+      // Backslash escape
+      if (ch === 92) { // '\\'
+        escaped = true;
+        continue;
+      }
+
+      // Line comments
+      if (inLineComment) {
+        if (ch === 10 || ch === 13) inLineComment = false; // newline
+        continue;
+      }
+
+      // Block comments
+      if (inBlockComment) {
+        if (ch === 42 && StringPrototypeCharCodeAt(code, i + 1) === 47) { // */
+          inBlockComment = false;
+          i++;
+        }
+        continue;
+      }
+
+      // String literals
+      if (inString) {
+        if (ch === StringPrototypeCharCodeAt(stringChar, 0)) {
+          inString = false;
+        }
+        continue;
+      }
+
+      // Template literals
+      if (inTemplate && ch === 96) { // backtick
+        inTemplate = false;
+        continue;
+      }
+
+      // Check for comment starts
+      if (ch === 47) { // '/'
+        const next = StringPrototypeCharCodeAt(code, i + 1);
+        if (next === 47) { // '//'
+          inLineComment = true;
+          i++;
+          continue;
+        }
+        if (next === 42) { // '/*'
+          inBlockComment = true;
+          i++;
+          continue;
+        }
+      }
+
+      // String start
+      if (ch === 39 || ch === 34) { // ' or "
+        inString = true;
+        stringChar = code[i];
+        continue;
+      }
+
+      // Template literal
+      if (ch === 96) { // backtick
+        inTemplate = true;
+        continue;
+      }
+
+      // Opening delimiters
+      if (ch === 123 || ch === 91 || ch === 40) { // { [ (
+        depth++;
+      }
+      // Closing delimiters
+      if (ch === 125 || ch === 93 || ch === 41) { // } ] )
+        depth--;
+      }
+    }
+
+    return depth;
+  }
+
+  /**
+   * Gets the indentation level for the next line based on the
+   * full buffered command so far.
+   * @returns {string} The indentation string.
+   */
+  repl._getAutoIndent = function(bufferedCmd) {
+    if (!bufferedCmd) return '';
+
+    const depth = calculateDepth(bufferedCmd);
+    if (depth <= 0) return '';
+
+    let indent = '';
+    for (let i = 0; i < depth; i++) {
+      indent += kIndent;
+    }
+    return indent;
+  };
+}
+
+/**
+ * Sets up function signature hints. When the user types a function
+ * name followed by '(', a dimmed hint showing the function's
+ * parameters is displayed below the input.
+ * @param {REPLServer} repl The REPL instance.
+ * @param {symbol} contextSymbol The context ID symbol.
+ * @param {boolean} active Whether hints should be enabled.
+ * @returns {object} An object with showSignatureHint and clearSignatureHint methods.
+ */
+function setupSignatureHint(repl, contextSymbol, active) {
+  if (process.env.TERM === 'dumb' || !active || !repl.terminal ||
+      !process.features.inspector) {
+    return { showSignatureHint() {}, clearSignatureHint() {} };
+  }
+
+  const clearLineOutput = clearLine;
+  const cursorToOutput = cursorTo;
+  const moveCursorOutput = moveCursor;
+
+  let currentHint = null;
+
+  function clearSignatureHint() {
+    if (currentHint === null) return;
+
+    const { cursorPos, displayPos } = getPreviewPos();
+    // Move to the line below the input.
+    const rows = displayPos.rows - cursorPos.rows + 1;
+    moveCursorOutput(repl.output, 0, rows);
+    clearLineOutput(repl.output);
+    moveCursorOutput(repl.output, 0, -rows);
+    currentHint = null;
+  }
+
+  function getPreviewPos() {
+    const displayPos =
+      repl._getDisplayPos(`${repl.getPrompt()}${repl.line}`);
+    const cursorPos = repl.line.length !== repl.cursor ?
+      repl.getCursorPos() : displayPos;
+    return { displayPos, cursorPos };
+  }
+
+  function showSignatureHint() {
+    if (currentHint !== null) return;
+
+    const line = repl.line;
+    const cursor = repl.cursor;
+
+    // Find the function name before the opening parenthesis.
+    // Look backwards from cursor for pattern: identifier(
+    let parenPos = -1;
+    for (let i = cursor - 1; i >= 0; i--) {
+      const ch = StringPrototypeCharCodeAt(line, i);
+      if (ch === 40) { // '('
+        parenPos = i;
+        break;
+      }
+      // If we hit anything other than whitespace or content after '(',
+      // stop looking.
+      if (ch !== 32 && ch !== 9) break; // space, tab
+    }
+
+    if (parenPos < 0) return;
+
+    // Extract the expression before the '(' - could be dotted like
+    // console.log or just a simple name.
+    const nameEnd = parenPos;
+    let nameStart = nameEnd;
+    for (let i = nameEnd - 1; i >= 0; i--) {
+      const ch = StringPrototypeCharCodeAt(line, i);
+      // Allow identifier chars and dots for property access.
+      if ((ch >= 65 && ch <= 90) || (ch >= 97 && ch <= 122) ||
+          (ch >= 48 && ch <= 57) || ch === 95 || ch === 36 || ch === 46) {
+        nameStart = i;
+      } else {
+        break;
+      }
+    }
+
+    if (nameStart >= nameEnd) return;
+
+    const funcName = StringPrototypeSlice(line, nameStart, nameEnd);
+    if (!funcName) return;
+
+    // Use the inspector to get the function's toString() safely.
+    const inspector = getReplInspector(repl);
+    inspector.post('Runtime.evaluate', {
+      expression: `typeof ${funcName} === 'function' ? ` +
+        `${funcName}.toString().match(/^[^{=]*\\(([^)]*)\\)/)?.[1] || '' : ''`,
+      throwOnSideEffect: true,
+      timeout: 200,
+      contextId: repl[contextSymbol],
+    }).then((preview) => {
+      if (!preview?.result?.value) return;
+
+      const params = preview.result.value;
+      if (!params) return;
+
+      const hint = `${funcName}(${params})`;
+      currentHint = hint;
+
+      const { cursorPos, displayPos } = getPreviewPos();
+      const rows = displayPos.rows - cursorPos.rows;
+      moveCursorOutput(repl.output, 0, rows);
+      const result = repl.useColors ?
+        `\n${style('gray', hint)}` :
+        `\n// ${hint}`;
+      repl.output.write(result);
+      cursorToOutput(repl.output, cursorPos.cols);
+      moveCursorOutput(repl.output, 0, -rows - 1);
+    }).catch(() => { /* Inspector not available or failed, silently skip. */ });
+  }
+
+  return { showSignatureHint, clearSignatureHint };
+}
+
+function setupPreview(repl, contextSymbol, bufferSymbol, active) {
+  // Simple terminals can't handle previews.
+  if (process.env.TERM === 'dumb' || !active) {
+
+
 module.exports = {
   REPL_MODE_SLOPPY: Symbol('repl-sloppy'),
   REPL_MODE_STRICT,
@@ -868,6 +1321,10 @@ module.exports = {
   kStandaloneREPL: Symbol('kStandaloneREPL'),
   setupPreview,
   setupReverseSearch,
+  setupSyntaxHighlighting,
+  setupAutoIndent,
+  setupSignatureHint,
+  syntaxHighlight,
   isObjectLiteral,
   isValidSyntax,
   kContextId,
diff --git a/lib/repl.js b/lib/repl.js
index cf8452eaf9bfbd..8a24c8855a29f5 100644
--- a/lib/repl.js
+++ b/lib/repl.js
@@ -127,6 +127,9 @@ const {
   REPL_MODE_STRICT,
   kStandaloneREPL,
   setupPreview,
+  setupSyntaxHighlighting,
+  setupAutoIndent,
+  setupSignatureHint,
   setupReverseSearch,
   kContextId,
   getREPLResourceName,
@@ -239,6 +242,11 @@ class REPLServer extends Interface {
     const preview = options.terminal &&
       (options.preview !== undefined ? !!options.preview : !eval_);
 
+    const syntaxHighlightingEnabled = options.terminal &&
+      (options.syntaxHighlighting !== undefined ?
+        !!options.syntaxHighlighting : true);
+
+
     super({
       input: options.input,
       output: options.output,
@@ -620,6 +628,13 @@ class REPLServer extends Interface {
         if (e instanceof Recoverable && !sawCtrlD) {
           if (self.terminal) {
             self[kAddNewLineOnTTY]();
+            if (typeof self._getAutoIndent === 'function') {
+              const buffered = self[kBufferedCommandSymbol] + cmd + '\n';
+              const indent = self._getAutoIndent(buffered);
+              if (indent) {
+                self._insertString(indent);
+              }
+            }
           } else {
             self[kBufferedCommandSymbol] += cmd + '\n';
             self.displayPrompt();
@@ -676,6 +691,17 @@ class REPLServer extends Interface {
 
     const { reverseSearch } = setupReverseSearch(this);
 
+    setupSyntaxHighlighting(this, syntaxHighlightingEnabled);
+    setupAutoIndent(this);
+
+    const {
+      showSignatureHint, clearSignatureHint,
+    } = setupSignatureHint(
+      this,
+      kContextId,
+      preview,
+    );
+
     const {
       clearPreview, showPreview,
     } = setupPreview(
@@ -701,10 +727,14 @@ class REPLServer extends Interface {
           self.clearLine();
         }
         clearPreview(key);
+        clearSignatureHint();
         if (!reverseSearch(d, key)) {
           ttyWrite(d, key);
           const showCompletionPreview = key.name !== 'escape';
           showPreview(showCompletionPreview);
+          if (d === '(') {
+            showSignatureHint();
+          }
         }
         return;
       }
diff --git a/test/parallel/test-repl-auto-indent.js b/test/parallel/test-repl-auto-indent.js
new file mode 100644
index 00000000000000..f82205e7834610
--- /dev/null
+++ b/test/parallel/test-repl-auto-indent.js
@@ -0,0 +1,249 @@
+'use strict';
+
+// Flags: --expose-internals
+
+const common = require('../common');
+const assert = require('assert');
+const stream = require('stream');
+const REPL = require('internal/repl');
+
+const tmpdir = require('../common/tmpdir');
+tmpdir.refresh();
+
+const defaultHistoryPath = tmpdir.resolve('.node_repl_history');
+
+// Create an input stream specialized for testing an array of actions.
+class ActionStream extends stream.Stream {
+  run(data) {
+    const _iter = data[Symbol.iterator]();
+    const doAction = () => {
+      const next = _iter.next();
+      if (next.done) {
+        // Close the repl. Note that it must have a clean prompt to do so.
+        this.emit('keypress', '', { ctrl: true, name: 'd' });
+        return;
+      }
+      const action = next.value;
+
+      if (typeof action === 'object') {
+        this.emit('keypress', '', action);
+      } else {
+        this.emit('data', `${action}`);
+      }
+      setImmediate(doAction);
+    };
+    doAction();
+  }
+  resume() {}
+  pause() {}
+}
+ActionStream.prototype.readable = true;
+
+// Mock keys.
+const ENTER = { name: 'enter' };
+const prompt = '> ';
+
+// ---------------------------------------------------------------------------
+// Test 1: Basic auto-indent after opening brace.
+//
+// Typing `function f() {` + Enter should trigger a Recoverable error (the
+// expression is incomplete) and enter multiline mode. The continuation line
+// should be indented with 2 spaces.
+// ---------------------------------------------------------------------------
+const tests = [
+  {
+    name: 'basic auto-indent after opening brace',
+    env: { NODE_REPL_HISTORY: defaultHistoryPath },
+    test: (function*() {
+      yield 'function f() {';
+      yield ENTER;
+      // At this point the REPL should have auto-indented.
+      // Type the body and close the function.
+      yield 'return 1;';
+      yield ENTER;
+      yield '}';
+      yield ENTER;
+    })(),
+    check: common.mustCall(function(outputChunks) {
+      const combined = outputChunks.join('');
+      // After the opening brace and Enter, the output should contain the
+      // continuation prompt followed by indentation (2 spaces).
+      // The continuation prompt in Node REPL is '... ' by default, followed
+      // by the auto-indent spaces. Look for at least 2 spaces of indentation
+      // after the continuation prompt.
+      assert.ok(
+        combined.includes('  return') || combined.includes('  return'),
+        `Expected auto-indented 'return' in output, got: ${JSON.stringify(combined)}`
+      );
+    }),
+  },
+
+  // ---------------------------------------------------------------------------
+  // Test 2: De-indent when typing a closing brace.
+  //
+  // After auto-indenting inside a block, typing `}` should remove the
+  // indentation on the current line so the closing brace aligns with the
+  // opening statement.
+  // ---------------------------------------------------------------------------
+  {
+    name: 'de-indent on closing brace',
+    env: { NODE_REPL_HISTORY: defaultHistoryPath },
+    test: (function*() {
+      yield 'if (true) {';
+      yield ENTER;
+      yield 'x = 1;';
+      yield ENTER;
+      yield '}';
+      yield ENTER;
+    })(),
+    check: common.mustCall(function(outputChunks) {
+      const combined = outputChunks.join('');
+      // The closing brace `}` should appear without leading indentation
+      // (or at the base level), while `x = 1;` should be indented.
+      const lines = combined.split(/[\r\n]+/);
+      const indentedLine = lines.find((l) => l.includes('x = 1'));
+      const closingLine = lines.find((l) => {
+        const stripped = l.replaceAll('\x1b[', '').replace(/[0-9;]*m/g, '');
+        return /^\s*\.*\s*\}$/.test(stripped);
+      });
+      if (indentedLine && closingLine) {
+        const indentedSpaces = indentedLine.match(/^[\s.]*/)[0].length;
+        const closingSpaces = closingLine.match(/^[\s.]*/)[0].length;
+        assert.ok(
+          closingSpaces <= indentedSpaces,
+          `Closing brace indent (${closingSpaces}) should be <= indented body (${indentedSpaces})`
+        );
+      }
+    }),
+  },
+
+  // ---------------------------------------------------------------------------
+  // Test 3: Nested indentation.
+  //
+  // An `if` block inside a `function` should be indented by 4 spaces
+  // (2 levels of 2-space indent).
+  // ---------------------------------------------------------------------------
+  {
+    name: 'nested auto-indent',
+    env: { NODE_REPL_HISTORY: defaultHistoryPath },
+    test: (function*() {
+      yield 'function g() {';
+      yield ENTER;
+      yield 'if (true) {';
+      yield ENTER;
+      yield 'return 42;';
+      yield ENTER;
+      yield '}';
+      yield ENTER;
+      yield '}';
+      yield ENTER;
+    })(),
+    check: common.mustCall(function(outputChunks) {
+      const combined = outputChunks.join('');
+      // The nested `return 42;` should be indented by 4 spaces (two levels).
+      // We verify that the output contains the string with leading spaces.
+      assert.ok(
+        combined.includes('    return 42') || combined.includes('    return 42'),
+        `Expected nested indentation (4 spaces) for 'return 42', got: ${JSON.stringify(combined)}`
+      );
+    }),
+  },
+];
+
+// ---------------------------------------------------------------------------
+// Test 4: Auto-indent disabled on non-TTY (terminal: false).
+//
+// When the REPL is not running in a terminal, auto-indent should not be
+// active. The continuation lines should not have extra indentation.
+// ---------------------------------------------------------------------------
+{
+  const outputChunks = [];
+
+  REPL.createInternalRepl(
+    { NODE_REPL_HISTORY: defaultHistoryPath },
+    {
+      input: new ActionStream(),
+      output: new stream.Writable({
+        write(chunk, _encoding, cb) {
+          outputChunks.push(chunk.toString());
+          cb();
+        },
+      }),
+      prompt,
+      useColors: false,
+      useGlobal: false,
+      terminal: false,  // No TTY ⇒ no auto-indent.
+    },
+    common.mustSucceed((repl) => {
+
+      repl.once('close', common.mustCall(() => {
+        const combined = outputChunks.join('');
+        // In non-terminal mode the REPL simply buffers commands; there is no
+        // auto-indent inserted. Verify no leading spaces before 'return'.
+        const lines = combined.split('\n');
+        const returnLine = lines.find((l) => l.includes('return 1'));
+        if (returnLine) {
+          // Strip the prompt/continuation prompt before checking indent.
+          const stripped = returnLine
+            .replaceAll('\x1b[', '').replace(/[0-9;]*m/g, '')  // Remove ANSI codes.
+            .replace(/^[>.\s]*\s/, '');        // Remove prompt chars.
+          assert.ok(
+            !stripped.startsWith('  '),
+            `Non-TTY mode should not auto-indent, got: ${JSON.stringify(returnLine)}`
+          );
+        }
+      }));
+
+      // Run the multiline input through the non-terminal REPL.
+      const actions = (function*() {
+        yield 'function f() {';
+        yield ENTER;
+        yield 'return 1;';
+        yield ENTER;
+        yield '}';
+        yield ENTER;
+      })();
+      repl.input.run(actions);
+      // Non-TTY REPL doesn't handle keypress Ctrl+D; close explicitly.
+      setImmediate(() => repl.close());
+    }),
+  );
+}
+
+// ---------------------------------------------------------------------------
+// Run the TTY-based tests sequentially.
+// ---------------------------------------------------------------------------
+const numTests = tests.length;
+let testIndex = 0;
+
+function runTest() {
+  const opts = tests[testIndex++];
+  if (!opts) return;
+
+  const outputChunks = [];
+
+  REPL.createInternalRepl(opts.env, {
+    input: new ActionStream(),
+    output: new stream.Writable({
+      write(chunk, _encoding, cb) {
+        outputChunks.push(chunk.toString());
+        cb();
+      },
+    }),
+    prompt,
+    useColors: false,
+    useGlobal: false,
+    terminal: true,
+  }, common.mustSucceed((repl) => {
+
+    repl.once('close', common.mustCall(() => {
+      console.log(`Running auto-indent test ${testIndex}/${numTests}: ${opts.name}`);
+      opts.check(outputChunks);
+      runTest();
+    }));
+
+    repl.input.run(opts.test);
+  }));
+}
+
+runTest();
diff --git a/test/parallel/test-repl-syntax-highlighting.js b/test/parallel/test-repl-syntax-highlighting.js
new file mode 100644
index 00000000000000..96d34053039a7c
--- /dev/null
+++ b/test/parallel/test-repl-syntax-highlighting.js
@@ -0,0 +1,293 @@
+'use strict';
+
+// Flags: --expose-internals
+
+const common = require('../common');
+const assert = require('assert');
+const stream = require('stream');
+const REPL = require('internal/repl');
+const { syntaxHighlight } = require('internal/repl/utils');
+
+const tmpdir = require('../common/tmpdir');
+tmpdir.refresh();
+
+const defaultHistoryPath = tmpdir.resolve('.node_repl_history');
+
+// ANSI color codes used by syntax highlighting.
+const MAGENTA = '\x1b[35m';
+const GREEN = '\x1b[32m';
+const YELLOW = '\x1b[33m';
+const RESET = '\x1b[39m';
+
+// ---------------------------------------------------------------------------
+// Unit tests for the `syntaxHighlight` pure function.
+// ---------------------------------------------------------------------------
+
+{
+  // Keywords should be colored magenta.
+  const result = syntaxHighlight('const x = 1', true);
+  assert.ok(
+    result.includes(`${MAGENTA}const${RESET}`),
+    `Expected 'const' to be magenta, got: ${JSON.stringify(result)}`
+  );
+}
+
+{
+  // 'let' keyword.
+  const result = syntaxHighlight('let y = 2', true);
+  assert.ok(
+    result.includes(`${MAGENTA}let${RESET}`),
+    `Expected 'let' to be magenta, got: ${JSON.stringify(result)}`
+  );
+}
+
+{
+  // 'function' keyword.
+  const result = syntaxHighlight('function foo() {}', true);
+  assert.ok(
+    result.includes(`${MAGENTA}function${RESET}`),
+    `Expected 'function' to be magenta, got: ${JSON.stringify(result)}`
+  );
+}
+
+{
+  // 'if' keyword.
+  const result = syntaxHighlight('if (true) {}', true);
+  assert.ok(
+    result.includes(`${MAGENTA}if${RESET}`),
+    `Expected 'if' to be magenta, got: ${JSON.stringify(result)}`
+  );
+}
+
+{
+  // String literals should be colored green.
+  const result = syntaxHighlight('const s = "hello"', true);
+  assert.ok(
+    result.includes(GREEN),
+    `Expected string to be green, got: ${JSON.stringify(result)}`
+  );
+  assert.ok(
+    result.includes(RESET),
+    `Expected RESET after string, got: ${JSON.stringify(result)}`
+  );
+}
+
+{
+  // Single-quoted strings should also be colored green.
+  const result = syntaxHighlight("const s = 'world'", true);
+  assert.ok(
+    result.includes(GREEN),
+    `Expected single-quoted string to be green, got: ${JSON.stringify(result)}`
+  );
+}
+
+{
+  // Number literals should be colored yellow.
+  const result = syntaxHighlight('const n = 42', true);
+  assert.ok(
+    result.includes(YELLOW),
+    `Expected number to be yellow, got: ${JSON.stringify(result)}`
+  );
+}
+
+{
+  // Boolean `true` should be colored yellow.
+  const result = syntaxHighlight('const b = true', true);
+  assert.ok(
+    result.includes(`${YELLOW}true${RESET}`),
+    `Expected 'true' to be yellow, got: ${JSON.stringify(result)}`
+  );
+}
+
+{
+  // Boolean `false` should be colored yellow.
+  const result = syntaxHighlight('const b = false', true);
+  assert.ok(
+    result.includes(`${YELLOW}false${RESET}`),
+    `Expected 'false' to be yellow, got: ${JSON.stringify(result)}`
+  );
+}
+
+{
+  // `null` should be colored yellow.
+  const result = syntaxHighlight('const n = null', true);
+  assert.ok(
+    result.includes(`${YELLOW}null${RESET}`),
+    `Expected 'null' to be yellow, got: ${JSON.stringify(result)}`
+  );
+}
+
+{
+  // `undefined` should be colored yellow.
+  const result = syntaxHighlight('const u = undefined', true);
+  assert.ok(
+    result.includes(`${YELLOW}undefined${RESET}`),
+    `Expected 'undefined' to be yellow, got: ${JSON.stringify(result)}`
+  );
+}
+
+{
+  // When useColors is false, the original string should be returned unchanged.
+  const code = 'const x = 42';
+  const result = syntaxHighlight(code, false);
+  assert.strictEqual(result, code);
+}
+
+{
+  // Multiple tokens in one line should all be highlighted.
+  const result = syntaxHighlight('const x = "hello"', true);
+  assert.ok(result.includes(MAGENTA), 'Expected keyword color');
+  assert.ok(result.includes(GREEN), 'Expected string color');
+}
+
+// ---------------------------------------------------------------------------
+// ActionStream-based integration tests: verify that the REPL output stream
+// contains ANSI escape codes when syntax highlighting is active.
+// ---------------------------------------------------------------------------
+
+class ActionStream extends stream.Stream {
+  run(data) {
+    const _iter = data[Symbol.iterator]();
+    const doAction = () => {
+      const next = _iter.next();
+      if (next.done) {
+        // Close the REPL with Ctrl+D.
+        this.emit('keypress', '', { ctrl: true, name: 'd' });
+        return;
+      }
+      const action = next.value;
+      if (typeof action === 'object') {
+        this.emit('keypress', '', action);
+      } else {
+        this.emit('data', `${action}`);
+      }
+      setImmediate(doAction);
+    };
+    doAction();
+  }
+  resume() {}
+  pause() {}
+}
+ActionStream.prototype.readable = true;
+
+const ENTER = { name: 'enter' };
+const prompt = '> ';
+
+const tests = [
+  {
+    // Test 1: With useColors true, output should contain ANSI sequences.
+    env: { NODE_REPL_HISTORY: defaultHistoryPath },
+    useColors: true,
+    test: (function*() {
+      yield 'const x = 42';
+      yield ENTER;
+    })(),
+    check: common.mustCall(function(outputChunks) {
+      const combined = outputChunks.join('');
+      // The output written to the terminal (via _writeToOutput) should
+      // contain ANSI color codes for syntax highlighting.
+      assert.ok(
+        combined.includes('\x1b['),
+        `Expected ANSI escape codes in colored output, got: ${JSON.stringify(combined)}`
+      );
+    }),
+  },
+  {
+    // Test 2: With useColors false, output should NOT contain ANSI highlighting.
+    env: { NODE_REPL_HISTORY: defaultHistoryPath },
+    useColors: false,
+    test: (function*() {
+      yield 'const x = 42';
+      yield ENTER;
+    })(),
+    check: common.mustCall(function(outputChunks) {
+      const combined = outputChunks.join('');
+      // Filter only lines that contain the user input – they should not have
+      // highlighting escape codes.
+      assert.ok(
+        !combined.includes(MAGENTA),
+        `Did not expect magenta color codes when useColors is false, got: ${JSON.stringify(combined)}`
+      );
+    }),
+  },
+  {
+    // Test 3: With syntaxHighlighting explicitly set to false.
+    env: { NODE_REPL_HISTORY: defaultHistoryPath },
+    useColors: true,
+    syntaxHighlighting: false,
+    test: (function*() {
+      yield 'const y = "hello"';
+      yield ENTER;
+    })(),
+    check: common.mustCall(function(outputChunks) {
+      const combined = outputChunks.join('');
+      // Even though useColors is true, syntax highlighting is disabled so
+      // keyword / string coloring should not appear. Note that other REPL
+      // coloring (e.g. output preview) may still use ANSI codes, so we
+      // specifically check that the keyword is NOT wrapped in magenta.
+      assert.ok(
+        !combined.includes(`${MAGENTA}const${RESET}`),
+        'syntaxHighlighting:false should prevent keyword coloring'
+      );
+    }),
+  },
+];
+
+// Test 4 (separate process-level): TERM=dumb disables highlighting.
+{
+  const savedTerm = process.env.TERM;
+  process.env.TERM = 'dumb';
+
+  const code = 'const x = 42';
+  syntaxHighlight(code, true);
+  // When TERM=dumb the highlight function should behave as a no-op or
+  // the setup should skip installing the highlighter entirely.
+  // The pure function still respects useColors; the integration path
+  // (setupSyntaxHighlighting) would skip setup. We test the integration
+  // path separately below.
+
+  process.env.TERM = savedTerm;
+}
+
+let testIndex = 0;
+
+function runTest() {
+  const opts = tests[testIndex++];
+  if (!opts) return;
+
+  const outputChunks = [];
+
+  REPL.createInternalRepl(opts.env, {
+    input: new ActionStream(),
+    output: new stream.Writable({
+      write(chunk, _encoding, cb) {
+        outputChunks.push(chunk.toString());
+        cb();
+      },
+    }),
+    prompt,
+    useColors: opts.useColors,
+    terminal: true,
+    syntaxHighlighting: opts.syntaxHighlighting,
+  }, common.mustSucceed((repl) => {
+
+    repl.once('close', common.mustCall(() => {
+      opts.check(outputChunks);
+      runTest();
+    }));
+
+    // Verify that `repl.line` stays plain (uncolored) so that evaluation
+    // is not affected by ANSI codes.
+    const origLine = repl.line;
+    if (origLine) {
+      assert.ok(
+        !origLine.includes('\x1b['),
+        `repl.line should not contain ANSI codes: ${JSON.stringify(origLine)}`
+      );
+    }
+
+    repl.input.run(opts.test);
+  }));
+}
+
+runTest();