Can Deno's std/log assign different log levels to console and file handlers?

Yes, Deno's std/log module supports assigning different minimum log levels to individual handlers. You can configure a console handler to show only warnings and errors while a file handler records everything from debug upward — all within a single logger instance.

How per-handler levels work

In std/log, filtering happens at two layers:

• Logger level — the minimum severity the logger will pass to any handler.
• Handler level — the minimum severity each handler will accept before writing to its destination.

The effective level for a given output is the more restrictive of the two. So if your logger is set to DEBUG and your console handler is set to WARNING, the console shows only warnings and errors, while a file handler set to DEBUG receives everything.

Configuration example

Using the setup() function, you define handlers with their own levels and then attach them to a logger:

import * as log from "jsr:@std/log";

await log.setup({
  handlers: {
    console: new log.handlers.ConsoleHandler("WARNING"),
    file: new log.handlers.FileHandler("./app.log", {
      level: "DEBUG",
    }),
  },
  loggers: {
    default: {
      level: "DEBUG",
      handlers: ["console", "file"],
    },
  },
});

log.debug("debug message");   // file only
log.info("info message");     // file only
log.warning("warning message"); // console + file
log.error("error message");   // console + file

Level names are case-insensitive strings ("DEBUG", "INFO", "WARNING", "ERROR") or numeric values. The logger level must be at least as permissive as your most verbose handler — here DEBUG — or the handler will never see lower-severity messages.

Verification

Run the script with deno run --allow-write --allow-read your_script.ts. Check that the console output contains only warnings and errors, then inspect app.log to confirm debug and info lines are present. You can also call log.getHandlers() to inspect the configured levels programmatically.

Version note

This behavior is stable in current std/log releases (e.g., jsr:@std/log 0.224+). If you import from deno.land/std/log, verify the version you pin matches the API described above.

No separate logger instances are needed — one logger with per-handler levels gives you the unified interface you want.