diagnostics_channel: capture console messages

doc/api/diagnostics_channel.md

@@ -1121,6 +1121,43 @@ While the diagnostics\_channel API is now considered stable, the built-in
 channels currently available are not. Each channel must be declared stable
 independently.
 
+#### Console
+
+`console.log`
+
+* `args` {any\[]}
+
+Emitted when `console.log()` is called. Receives and array of the arguments
+passed to `console.log()`.
+
+`console.info`
+
+* `args` {any\[]}
+
+Emitted when `console.info()` is called. Receives and array of the arguments
+passed to `console.info()`.
+
+`console.debug`
+
+* `args` {any\[]}
+
+Emitted when `console.debug()` is called. Receives and array of the arguments
+passed to `console.debug()`.
+
+`console.warn`
+
+* `args` {any\[]}
+
+Emitted when `console.warn()` is called. Receives and array of the arguments
+passed to `console.warn()`.
+
+`console.error`
+
+* `args` {any\[]}
+
+Emitted when `console.error()` is called. Receives and array of the arguments
+passed to `console.error()`.
+
 #### HTTP
 
 `http.client.request.created`

lib/internal/console/constructor.js

@@ -61,6 +61,13 @@ const {
 } = require('internal/constants');
 const kCounts = Symbol('counts');
 const { time, timeLog, timeEnd, kNone } = require('internal/util/debuglog');
+const { channel } = require('diagnostics_channel');
+
+const onLog = channel('console.log');
+const onWarn = channel('console.warn');
+const onError = channel('console.error');
+const onInfo = channel('console.info');
+const onDebug = channel('console.debug');
 
 const kTraceConsoleCategory = 'node,node.console';
 
@@ -371,14 +378,39 @@ function timeLogImpl(consoleRef, label, formatted, args) {
 
 const consoleMethods = {
   log(...args) {
+    if (onLog.hasSubscribers) {
+      onLog.publish(args);
+    }
     this[kWriteToConsole](kUseStdout, this[kFormatForStdout](args));
   },
 
+  info(...args) {
+    if (onInfo.hasSubscribers) {
+      onInfo.publish(args);
+    }
+    this[kWriteToConsole](kUseStdout, this[kFormatForStdout](args));
+  },
+
+  debug(...args) {
+    if (onDebug.hasSubscribers) {
+      onDebug.publish(args);
+    }
+    this[kWriteToConsole](kUseStdout, this[kFormatForStdout](args));
+  },
 
   warn(...args) {
+    if (onWarn.hasSubscribers) {
+      onWarn.publish(args);
+    }
     this[kWriteToConsole](kUseStderr, this[kFormatForStderr](args));
   },
 
+  error(...args) {
+    if (onError.hasSubscribers) {
+      onError.publish(args);
+    }
+    this[kWriteToConsole](kUseStderr, this[kFormatForStderr](args));
+  },
 
   dir(object, options) {
     this[kWriteToConsole](kUseStdout, inspect(object, {
@@ -614,10 +646,7 @@ function noop() {}
 for (const method of ReflectOwnKeys(consoleMethods))
   Console.prototype[method] = consoleMethods[method];
 
-Console.prototype.debug = Console.prototype.log;
-Console.prototype.info = Console.prototype.log;
 Console.prototype.dirxml = Console.prototype.log;
-Console.prototype.error = Console.prototype.warn;
 Console.prototype.groupCollapsed = Console.prototype.group;
 
 function initializeGlobalConsole(globalConsole) {

test/parallel/test-console-diagnostics-channels.js

@@ -0,0 +1,77 @@
+'use strict';
+
+const { mustCall } = require('../common');
+const { deepStrictEqual, ok, strictEqual } = require('assert');
+
+const { channel } = require('diagnostics_channel');
+
+const {
+  hijackStdout,
+  hijackStderr,
+  restoreStdout,
+  restoreStderr
+} = require('../common/hijackstdio');
+
+const stdoutMethods = [
+  'log',
+  'info',
+  'debug',
+];
+
+const stderrMethods = [
+  'warn',
+  'error',
+];
+
+const methods = [
+  ...stdoutMethods,
+  ...stderrMethods,
+];
+
+const channels = {
+  log: channel('console.log'),
+  info: channel('console.info'),
+  debug: channel('console.debug'),
+  warn: channel('console.warn'),
+  error: channel('console.error')
+};
+
+process.stdout.isTTY = false;
+process.stderr.isTTY = false;
+
+for (const method of methods) {
+  let intercepted = false;
+  let formatted = false;
+
+  const isStdout = stdoutMethods.includes(method);
+  const hijack = isStdout ? hijackStdout : hijackStderr;
+  const restore = isStdout ? restoreStdout : restoreStderr;
+
+  const foo = 'string';
+  const bar = { key: /value/ };
+  const baz = [ 1, 2, 3 ];
+
+  channels[method].subscribe(mustCall((args) => {
+    // Should not have been formatted yet.
+    intercepted = true;
+    ok(!formatted);
+
+    // Should receive expected log message args.
+    deepStrictEqual(args, [foo, bar, baz]);
+
+    // Should be able to mutate message args and have it reflected in output.
+    bar.added = true;
+  }));
+
+  hijack(mustCall((output) => {
+    // Should have already been intercepted.
+    formatted = true;
+    ok(intercepted);
+
+    // Should produce expected formatted output with mutated message args.
+    strictEqual(output, 'string { key: /value/, added: true } [ 1, 2, 3 ]\n');
+  }));
+
+  console[method](foo, bar, baz);
+  restore();
+}