Why JavaScript treats a number as octal if it has a leading zero

by

I think my answer here answers the question, but the question is not exactly a duplicate, so I include a copy of my answer.

History

The problem is that decimal integer literals can’t have leading zeros:

DecimalIntegerLiteral ::
    0
    NonZeroDigit DecimalDigits(opt)

However, ECMAScript 3 allowed (as an optional extension) to parse literals with leading zeros in base 8:

OctalIntegerLiteral ::
    0 OctalDigit
    OctalIntegerLiteral OctalDigit

But ECMAScript 5 forbade doing that in strict-mode:

A conforming implementation, when processing strict mode code (see
10.1.1), must not extend the syntax of NumericLiteral to include OctalIntegerLiteral as described in B.1.1.

ECMAScript 6 introduces BinaryIntegerLiteral and OctalIntegerLiteral, so now we have more coherent literals:

  • BinaryIntegerLiteral, prefixed with 0b or 0B.
  • OctalIntegerLiteral, prefixed with 0o or 0O.
  • HexIntegerLiteral, prefixed with 0x or 0X.

The old OctalIntegerLiteral extension has been renamed to LegacyOctalIntegerLiteral, which is still allowed in non-strict mode.

Conclusion

Therefore, if you want to parse a number in base 8, use the 0o or 0O prefixes (not supported by old browsers), or use parseInt.

And if you want to be sure your numbers will be parsed in base 10, remove leading zeros, or use parseInt.

Examples

  • 010
    • In strict mode (requires ECMAScript 5), it throws.
    • In non strict mode, it may throw or return 8 (implementation dependent).
  • 0o10, 0O10
    • Before ECMAScript 6, they throw.
    • In ECMAScript 6, they return 8.
  • parseInt('010', 8)
    • It returns 8.
  • parseInt('010', 10)
    • It returns 10.

More Related Contents:

  1. Set a default parameter value for a JavaScript function
  2. Capture HTML Canvas as gif/jpg/png/pdf?
  3. State not updating when using React state hook within setInterval
  4. How can I pass variable into an evaluate function?
  5. Copy array by value
  6. Calling member function of number literal
  7. AngularJs – cancel route change event
  8. Access CSS variable from javascript [duplicate]
  9. Why does 2 == [2] in JavaScript?
  10. Why is null in JavaScript bigger than -1, less than 1, but not equal (==) to 0? What is it exactly then?
  11. addEventListener on a querySelectorAll() with classList [duplicate]
  12. Get element by part of Name or ID
  13. How can I benchmark JavaScript code? [closed]
  14. React useEffect causing: Can’t perform a React state update on an unmounted component
  15. Is there some innerHTML replacement in SVG/XML?
  16. Angularjs: ‘controller as syntax’ and $watch
  17. Is it possible to execute JSX scripts from outside ExtendScript?
  18. Testing if jQueryUI has loaded
  19. Dark mode flickers a white background for a millisecond on reload
  20. Delete from array in javascript
  21. Splicing a Javascript array from within the callback passed to forEach
  22. How to Transpose / Rotate Multi-dimensional Array?
  23. Keep only first n characters in a string?
  24. When does async function actually return a pending promise?
  25. Sorting a list by data-attribute
  26. What is the order of evaluation for function arguments in Javascript?
  27. Is there an equivalent of canvas’s toDataURL method for SVG? [closed]
  28. JavaScript object literal length === undefined?
  29. Capturing result of window.onbeforeunload confirmation dialog
  30. Split text into pages and present separately (HTML5)

Leave a Comment