doc: warn about using util.inspect/util.format

Because of the potential performance bottlenecks that may be introduced
by `util.inspect()` and `util.format()` in production hot path code.

Based on real user feedback, it is not obvious that these are intended
to be debugging tools.

PR-URL: https://github.com/nodejs/node/pull/17791
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/doc/api/util.md b/doc/api/util.md
index 8f9e6ef020..a2d3c71bb6 100644
--- a/doc/api/util.md
+++ b/doc/api/util.md
@@ -250,6 +250,11 @@ without any formatting.
 util.format('%% %s'); // '%% %s'
 ```
 
+Please note that `util.format()` is a synchronous method that is mainly
+intended as a debugging tool. Some input values can have a significant
+performance overhead that can block the event loop. Use this function
+with care and never in a hot code path.
+
 ## util.inherits(constructor, superConstructor)
 <!-- YAML
 added: v0.3.0
@@ -462,6 +467,11 @@ console.log(util.inspect(o, { compact: false, depth: 5, breakLength: 80 }));
 // chunks.
 ```
 
+Please note that `util.inspect()` is a synchronous method that is mainly
+intended as a debugging tool. Some input values can have a significant
+performance overhead that can block the event loop. Use this function
+with care and never in a hot code path.
+
 ### Customizing `util.inspect` colors
 
 <!-- type=misc -->