test: actually test tty getColorDepth()

test/common/README.md

@@ -679,6 +679,12 @@ The realpath of the testing temporary directory.
 
 Deletes and recreates the testing temporary directory.
 
+### getTTYfd()
+
+Attempts to get a valid TTY file descriptor. Returns `-1` if it fails.
+
+The TTY file descriptor is assumed to be capable of being writable.
+
 ## WPT Module
 
 The wpt.js module is a port of parts of

test/common/index.js

@@ -775,6 +775,23 @@ exports.crashOnUnhandledRejection = function() {
              (err) => process.nextTick(() => { throw err; }));
 };
 
+exports.getTTYfd = function getTTYfd() {
+  // Do our best to grab a tty fd.
+  const tty = require('tty');
+  // Don't attempt fd 0 as it is not writable on Windows.
+  // Ref: ef2861961c3d9e9ed6972e1e84d969683b25cf95
+  const ttyFd = [1, 2, 4, 5].find(tty.isatty);
+  if (ttyFd === undefined) {
+    try {
+      return fs.openSync('/dev/tty');
+    } catch (e) {
+      // There aren't any tty fd's available to use.
+      return -1;
+    }
+  }
+  return ttyFd;
+};
+
 // Hijack stdout and stderr
 const stdWrite = {};
 function hijackStdWritable(name, listener) {

test/pseudo-tty/test-tty-get-color-depth.js

@@ -0,0 +1,36 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert').strict;
+/* eslint-disable no-restricted-properties */
+const { WriteStream } = require('tty');
+
+const fd = common.getTTYfd();
+const writeStream = new WriteStream(fd);
+
+{
+  const depth = writeStream.getColorDepth();
+
+  assert.equal(typeof depth, 'number');
+  assert(depth >= 1 && depth <= 24);
+
+  if (depth === 1) {
+    // Terminal does not support colors, compare to a value that would.
+    assert.notEqual(writeStream.getColorDepth({ COLORTERM: '1' }), depth);
+  } else {
+    // Terminal supports colors, compare to a value that would not.
+    assert.notEqual(writeStream.getColorDepth({ TERM: 'dumb' }), depth);
+  }
+}
+
+// Deactivate colors
+{
+  const tmp = process.env.NODE_DISABLE_COLORS;
+  process.env.NODE_DISABLE_COLORS = 1;
+
+  const depth = writeStream.getColorDepth();
+
+  assert.equal(depth, 1);
+
+  process.env.NODE_DISABLE_COLORS = tmp;
+}