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fix: prevent re-interpretation of % in formatted output #1028

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fix: prevent re-interpretation of % in formatted output #1028

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fix: prevent re-interpretation of % in formatted output (#766)
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@abhu85
abhu85 committed Apr 29, 2026
commit 3e7da05bea24135513c9fc72e98ba27cb940ad3e
10 changes: 7 additions & 3 deletions src/common.js
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Original file line number Diff line number Diff line change
Expand Up @@ -99,9 +99,13 @@ function setup(env) {
const val = args[index];
match = formatter.call(self, val);

// Now we need to remove `args[index]` since it's inlined in the `format`
args.splice(index, 1);
index--;
// Replace the original argument with the formatted string
// and use %s as a placeholder so that any `%` characters in
// the formatted output are not re-interpreted as format
// specifiers by the downstream logger (Node's
// `util.formatWithOptions` or the browser console).
args[index] = match;
match = '%s';
}
return match;
});
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74 changes: 74 additions & 0 deletions test.js
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Expand Up @@ -80,6 +80,80 @@ describe('debug', () => {
});
});

describe('percent signs in formatted output (#766, #687)', () => {
it('should not re-interpret % in object keys/values with %o', () => {
const log = debug('test');
log.enabled = true;

const messages = [];
log.log = (...args) => messages.push(args);

log('%o', {'%j': '%j'});

assert.strictEqual(messages.length, 1);
const output = messages[0].join(' ');
assert.ok(output.includes('%j'), 'should contain literal %j in output');
});

it('should preserve percent in values with %o', () => {
const log = debug('test');
log.enabled = true;

const messages = [];
log.log = (...args) => messages.push(args);

log('%o', {key: '100%'});

assert.strictEqual(messages.length, 1);
const output = messages[0].join(' ');
assert.ok(output.includes('100%'), 'should contain literal 100% in output');
});

it('should handle mixed specifiers with % in objects', () => {
const log = debug('test');
log.enabled = true;

const messages = [];
log.log = (...args) => messages.push(args);

log('%s %o', 'hello', {'%d': 1});

assert.strictEqual(messages.length, 1);
const output = messages[0].join(' ');
assert.ok(output.includes('hello'), 'should contain hello');
assert.ok(output.includes('%d'), 'should contain literal %d in object');
});

it('should still format normal objects with %o', () => {
const log = debug('test');
log.enabled = true;

const messages = [];
log.log = (...args) => messages.push(args);

log('%o', {a: 1, b: 'test'});

assert.strictEqual(messages.length, 1);
const output = messages[0].join(' ');
assert.ok(output.includes('a'), 'should contain key a');
assert.ok(output.includes('test'), 'should contain value test');
});

it('should not let %j in data consume extra arguments (#687)', () => {
const log = debug('test');
log.enabled = true;

const messages = [];
log.log = (...args) => messages.push(args);

log('%o', ['%j']);

assert.strictEqual(messages.length, 1);
const output = messages[0].join(' ');
assert.ok(output.includes('%j'), 'should contain literal %j in array output');
});
});

describe('rebuild namespaces string (disable)', () => {
it('handle names, skips, and wildcards', () => {
debug.enable('test,abc*,-abc');
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