Great question—this is a common point of confusion with Winston, so let's break it down clearly.

First, the top-level level is a global filter that acts as the first gatekeeper for all logs, while transport-specific level settings are additional filters applied after the global one. They don't override each other—they work together in a layered way.

Here's the exact flow: #

1. Global Filter Check: Every log message you send first gets evaluated against the top-level level. Only logs with a level less than or equal to the top-level value will move forward to be processed by your transports.
   • In your example, the top-level level: 'info' means debug (and any lower-priority levels) are immediately discarded—they never even reach your file transports, no matter what level those transports are set to.
2. Transport-Specific Filter Check: For logs that pass the global filter, each transport then applies its own level filter (if set). Again, only logs with a level less than or equal to the transport's level get written by that transport.
   • Your error.log transport uses level: 'error', so only error-level logs (which passed the global info check) are written here.
   • Your combined.log transport doesn't have a custom level, so it falls back to using the top-level level: 'info'—meaning all logs that passed the global filter (error, warn, info) get written here.

What happens if you skip the top-level level? #

If you don't set a top-level level, Winston defaults to info anyway. But explicitly setting it makes your configuration clearer, and lets you easily adjust the global log threshold for all transports at once.

Key Takeaway #

• The top-level level sets the minimum priority for logs to be processed at all.
• Transport-specific levels let you narrow down which logs each transport handles, without affecting others.
• They don't override each other—think of it as a two-step filter: global first, then per-transport.

内容的提问来源于stack exchange，提问作者Alexander Solonik