feat(consoleFormat): resolve console template specifiers in messages

src/formatters/ConsoleFormatter.ts

@@ -12,6 +12,7 @@ import {
 import {UncaughtError} from '../PageCollector.js';
 import * as DevTools from '../third_party/index.js';
 import type {ConsoleMessage} from '../third_party/index.js';
+import {formatConsoleMessage} from '../utils/ConsoleFormat.js';
 
 export interface ConsoleFormatterOptions {
   fetchDetailedData?: boolean;
@@ -36,6 +37,7 @@ export class ConsoleFormatter {
 
   readonly #argCount: number;
   readonly #resolvedArgs: unknown[];
+  readonly #remainingArgs: unknown[];
 
   readonly #stack?: DevTools.DevTools.StackTrace.StackTrace.StackTrace;
   readonly #cause?: SymbolizedError;
@@ -57,6 +59,17 @@ export class ConsoleFormatter {
     this.#text = params.text;
     this.#argCount = params.argCount ?? 0;
     this.#resolvedArgs = params.resolvedArgs ?? [];
+
+    // Calculate remaining arguments after template substitution
+    if (this.#resolvedArgs.length > 0 && this.#text) {
+      const {remainingArgs} = formatConsoleMessage(this.#text, this.#resolvedArgs);
+      this.#remainingArgs = remainingArgs;
+    } else if (!this.#text && this.#resolvedArgs.length > 0) {
+      this.#remainingArgs = this.#resolvedArgs.slice(1);
+    } else {
+      this.#remainingArgs = this.#resolvedArgs;
+    }
+
     this.#stack = params.stack;
     this.#cause = params.cause;
     this.#isIgnored = params.isIgnored;
@@ -112,12 +125,14 @@ export class ConsoleFormatter {
     let resolvedArgs: unknown[] = [];
     if (options.resolvedArgsForTesting) {
       resolvedArgs = options.resolvedArgsForTesting;
-    } else if (options.fetchDetailedData) {
+    } else {
       resolvedArgs = await Promise.all(
         msg.args().map(async (arg, i) => {
           try {
             const remoteObject = arg.remoteObject();
             if (
+              options.fetchDetailedData &&
+              options.devTools &&
               remoteObject.type === 'object' &&
               remoteObject.subtype === 'error'
             ) {
@@ -160,14 +175,14 @@ export class ConsoleFormatter {
 
   // The short format for a console message.
   toString(): string {
-    return `msgid=${this.#id} [${this.#type}] ${this.#text} (${this.#argCount} args)`;
+    return `msgid=${this.#id} [${this.#type}] ${this.#getFormattedText()} (${this.#argCount} args)`;
   }
 
   // The verbose format for a console message, including all details.
   toStringDetailed(): string {
     const result = [
       `ID: ${this.#id}`,
-      `Message: ${this.#type}> ${this.#text}`,
+      `Message: ${this.#type}> ${this.#getFormattedText()}`,
       this.#formatArgs(),
       this.#formatStackTrace(this.#stack, this.#cause, {
         includeHeading: true,
@@ -176,16 +191,22 @@ export class ConsoleFormatter {
     return result.join('\n');
   }
 
-  #getArgs(): unknown[] {
-    if (this.#resolvedArgs.length > 0) {
-      const args = [...this.#resolvedArgs];
-      // If there is no text, the first argument serves as text (see formatMessage).
-      if (!this.#text) {
-        args.shift();
-      }
-      return args;
+  // Gets the formatted message text, applying template string substitutions if arguments are available.
+  #getFormattedText(): string {
+    if (this.#resolvedArgs.length > 0 && this.#text) {
+      // apply template formatting
+      const {formattedText} = formatConsoleMessage(this.#text, this.#resolvedArgs);
+      return formattedText;
+    } else if (!this.#text && this.#resolvedArgs.length > 0) {
+      return this.#formatArg(this.#resolvedArgs[0]);
     }
-    return [];
+    // return the raw text
+    return this.#text;
+  }
+
+  #getArgs(): unknown[] {
+    // Return the remaining arguments after template substitution
+    return this.#remainingArgs;
   }
 
   #formatArg(arg: unknown) {

src/utils/ConsoleFormat.ts

@@ -0,0 +1,154 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * This utility formats console messages with template strings, reusing Chrome DevTools
+ * Based on the Console Standard (https://console.spec.whatwg.org/#formatter).
+ */
+
+// Formats a console message text with its arguments, resolving format specifiers.
+export function formatConsoleMessage(
+  text: string,
+  args: unknown[],
+): {formattedText: string; remainingArgs: unknown[]} {
+  if (!text) {
+    return {formattedText: text, remainingArgs: args};
+  }
+
+  let result = '';
+  let argIndex = 0;
+
+  // eslint-disable-next-line no-control-regex
+  const re = /%([%_Oocsdfi])|\x1B\[([\d;]*)m/g;
+
+  let lastIndex = 0;
+  let match: RegExpExecArray | null;
+
+  while ((match = re.exec(text)) !== null) {
+    result += text.substring(lastIndex, match.index);
+    lastIndex = re.lastIndex;
+
+    const specifier = match[1];
+
+    if (specifier !== undefined) {
+      switch (specifier) {
+        case '%':
+          // Escaped percent sign
+          result += '%';
+          break;
+
+        case 's':
+          // String substitution
+          if (argIndex < args.length) {
+            result += formatArg(args[argIndex++], 'string');
+          } else {
+            result += match[0]; // Keep the specifier if no arg available
+          }
+          break;
+
+        case 'c':
+          // Style substitution
+          if (argIndex < args.length) {
+            argIndex++; 
+          } else {
+            result += match[0]; 
+          }
+          break;
+
+        case 'o':
+        case 'O':
+          // Object substitution
+          if (argIndex < args.length) {
+            result += formatArg(args[argIndex++], 'object');
+          } else {
+            result += match[0]; 
+          }
+          break;
+
+        case '_':
+          // Ignore substitution
+          if (argIndex < args.length) {
+            argIndex++;
+          } else {
+            result += match[0];
+          }
+          break;
+
+        case 'd':
+        case 'i':
+          // Integer substitution
+          if (argIndex < args.length) {
+            const value = args[argIndex++];
+            const numValue =
+              typeof value === 'number' ? value : Number(value);
+            result += isNaN(numValue)
+              ? 'NaN'
+              : Math.floor(numValue).toString();
+          } else {
+            result += match[0]; 
+          }
+          break;
+
+        case 'f':
+          // Float substitution
+          if (argIndex < args.length) {
+            const value = args[argIndex++];
+            const numValue =
+              typeof value === 'number' ? value : Number(value);
+            result += isNaN(numValue) ? 'NaN' : numValue.toString();
+          } else {
+            result += match[0]; 
+          }
+          break;
+
+        default:
+          // Unknown specifier, keep it as is
+          result += match[0];
+          break;
+      }
+    } else {
+      // Handle ANSI escape codes - we ignore them in the formatted output
+    }
+  }
+
+  // Add any remaining text after the last match
+  result += text.substring(lastIndex);
+
+  // Return formatted text and unused arguments
+  return {
+    formattedText: result,
+    remainingArgs: args.slice(argIndex),
+  };
+}
+
+// Formats an argument value for display.
+function formatArg(arg: unknown, _hint: 'string' | 'object'): string {
+  if (arg === null) {
+    return 'null';
+  }
+
+  if (arg === undefined) {
+    return 'undefined';
+  }
+
+  if (typeof arg === 'string') {
+    return arg;
+  }
+
+  if (typeof arg === 'number' || typeof arg === 'boolean') {
+    return String(arg);
+  }
+
+  if (typeof arg === 'object') {
+    try {
+      return JSON.stringify(arg);
+    } catch {
+      return String(arg);
+    }
+  }
+
+  return String(arg);
+}