Summary

Prevent % characters in formatted object output from being re-interpreted as format specifiers.

Problem

When %o or %O formats an object containing % in its keys or values, the stringified output is passed directly into args[0]. The downstream logger (util.formatWithOptions in Node.js, or console.log in browsers) then re-scans the formatted string for %s, %d, %j, etc. specifiers found inside the already-formatted object, consuming subsequent arguments and producing garbled output.

For example:

debug('%o', { '%j': '%j %j %%' }, 1, 2, 3);
// Before: test { '1': '2 3 %' }   (garbled — %j consumed args 1, 2)
// After:  test { '%j': '%j %j %%' } 1 2 3   (correct)

Solution

Instead of inlining the formatter output directly into args[0] and removing the original argument, we now replace the original argument in-place with the formatted string and substitute %s as a placeholder in the format string. This delegates the final substitution to the downstream logger (util.formatWithOptions / console.log), which processes %s by consuming the next argument — the pre-formatted string — without re-scanning its contents for additional specifiers.

This is a minimal change (3 lines replaced) that preserves backward compatibility with all existing format specifiers.

Test Plan

• debug('%o', {'%j': '%j'}) outputs object literally
• debug('%o', {'key': '100%'}) preserves the percent
• Mixed specifiers with % in objects work correctly
• Existing format specifier tests pass (16 pre-existing + 5 new)
• Normal %o / %O formatting unchanged for objects without %

Fixes #766
Fixes #687