octalNumbers

Reports using legacy octal numeric literals.

✅ This rule is included in the ts untyped presets.

Legacy octal numeric literals (e.g., 077, 0123) are a deprecated feature in JavaScript that can lead to confusion and errors. They are forbidden in strict mode and are less readable than their modern alternatives. The explicit octal syntax 0o (e.g., 0o77) introduced in ES6 is clearer and works in both strict and non-strict modes. The digit 0 by itself is allowed as it represents zero, not an octal literal.

This rule reports on numeric literals with a preceding 0.

Examples

❌ Incorrect

const permissions = 0755; // Evaluates to 493, not 755!
Error ts(1121)  ― Octal literals are not allowed. Use the syntax '0o755'.
const value = 077; // Evaluates to 63, not 77!
Error ts(1121)  ― Octal literals are not allowed. Use the syntax '0o77'.
const numbers = [01, 02, 03, 04, 05, 06, 07];
Error ts(1121)  ― Octal literals are not allowed. Use the syntax '0o1'.
Error ts(1121)  ― Octal literals are not allowed. Use the syntax '0o2'.
Error ts(1121)  ― Octal literals are not allowed. Use the syntax '0o3'.
Error ts(1121)  ― Octal literals are not allowed. Use the syntax '0o4'.
Error ts(1121)  ― Octal literals are not allowed. Use the syntax '0o5'.
Error ts(1121)  ― Octal literals are not allowed. Use the syntax '0o6'.
Error ts(1121)  ― Octal literals are not allowed. Use the syntax '0o7'.

✅ Correct

// Use explicit octal syntax if you intended octal
const permissions = 0o755; // Clearly octal: 493 in decimal
Error ts(2451)  ― Cannot redeclare block-scoped variable 'permissions'.
const value = 0o77; // Clearly octal: 63 in decimal
Error ts(2451)  ― Cannot redeclare block-scoped variable 'value'.
const numbers = [0o1, 0o2, 0o3, 0o4, 0o5, 0o6, 0o7];

// Or use decimal if you meant the decimal value
const permissions = 755; // Clearly decimal
Error ts(2451)  ― Cannot redeclare block-scoped variable 'permissions'.
const value = 77; // Clearly decimal
Error ts(2451)  ― Cannot redeclare block-scoped variable 'value'.

// The number zero is allowed
const zero = 0;

// Other number formats are allowed
const decimal = 123;
const hex = 0xff;
const binary = 0b1010;

Options

This rule is not configurable.

When Not To Use It

If you need to maintain compatibility with very old JavaScript engines that don’t support the explicit octal syntax (ES6+), you might choose to disable this rule. However, this is extremely rare in modern development, as ES6 has been widely supported since 2015.

Further Reading

• ECMAScript Specification: Numeric Literals
• MDN: Numeric literals
• MDN: Strict Mode in JavaScript

Equivalents in Other Linters

• ESLint: no-octal