no-octal

A number that starts with 0 followed by more digits is read in base 8, so 071 is 57. If one of the digits is an 8 or a 9, it's read in base 10 after all, so 081 is 81. Write 0o71 if you mean octal, and drop the zero if you don't.

These numbers are a syntax error in modules and in strict mode. .js files are linted as modules, so the rule only has something to report in .cjs files, or where your config sets sourceType to "commonjs" or "script".

This rule is on by default.

Configure

On by default as a warning. To turn it off:

[lint.rules]
"no-octal" = "off" # or "error"

Examples

Reported:

const mode = 0755;
const month = 09;

Not reported:

const mode = 0o755;
const month = 9;
const ratio = 0.5;