util: runtime deprecation for custom .inspect()

Change documentation-only deprecation for custom inspection using
`object.inspect` property to a runtime deprecation.

This is a breaking change. Custom inspection via `object.inspect` is
deprecated because there is a more robust Symbol-based alternative to
`.inspect` and the custom inspection via `object.inspect` feature means
that people can accidentally break `console.log()` simply by attaching a
`.inspect` property to their objects. Note that since this is a
deprecation, the custom inspection will still work. The breaking change
is simply the printing of a warning which could alarm users, break tests
or other things that might be dependent on specific output, etc.

PR-URL: https://github.com/nodejs/node/pull/16393
Ref: https://github.com/nodejs/node/issues/15549
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

2 files changed, 24 insertions, 0 deletions

diff --git a/test/parallel/test-util-inspect-deprecated.js b/test/parallel/test-util-inspect-deprecated.js
new file mode 100644
index 0000000000..adabb06697
--- /dev/null
+++ b/test/parallel/test-util-inspect-deprecated.js
@@ -0,0 +1,18 @@
+'use strict';
+const common = require('../common');
+
+// Test that deprecation warning for custom inspection via the `.inspect()`
+// property (on the target object) is emitted once and only once.
+
+const util = require('util');
+
+{
+  const target = { inspect: () => 'Fhqwhgads' };
+  // `common.expectWarning` will expect the warning exactly one time only
+  common.expectWarning(
+    'DeprecationWarning',
+    'Custom inspection function on Objects via .inspect() is deprecated'
+  );
+  util.inspect(target);  // should emit deprecation warning
+  util.inspect(target);  // should not emit deprecation warning
+}
diff --git a/test/parallel/test-util-inspect.js b/test/parallel/test-util-inspect.js
index 2804ca4fb1..edd097ee7a 100644
--- a/test/parallel/test-util-inspect.js
+++ b/test/parallel/test-util-inspect.js
@@ -1150,3 +1150,9 @@ if (typeof Symbol !== 'undefined') {
 }
 
 assert.doesNotThrow(() => util.inspect(process));
+
+// Setting custom inspect property to a non-function should do nothing.
+{
+  const obj = { inspect: 'fhqwhgads' };
+  assert.strictEqual(util.inspect(obj), "{ inspect: 'fhqwhgads' }");
+}