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var x = 010; console.log(x); //8

JS engine convert the number x to octal number. Why it happens? How can I prevent it?

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    @Andy - 10, not 8. He's asking why prefixing a number with a 0 makes it assume it's octal. I don't blame him for asking. Commented May 3, 2016 at 12:12
  • A literal numeric value that starts with a 0 is always interpreted as an octal value. Commented May 3, 2016 at 12:13
  • Hah, yeah, I didn't ask that question properly. I know why it logs it as octal, I just wondered if 10 was the expected output from the OP. Commented May 3, 2016 at 12:14
  • 1
    If we prefix a number with 0x then it will convert the number to Hexadecimal as well Commented May 4, 2016 at 4:53
  • 1
    Does this answer your question? Number with leading zero in JavaScript Commented May 31, 2022 at 17:28

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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.
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Comments

2

It's because some JavaScript engines interpret leading zeros as octal number literals. It is defined in an appendix of ECMAScript specification.

However, in strict mode, the conforming implementations must not implement that - see ECMAScript specification again:

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.

Because of this ambiguity, it's better not to use leading zeros.

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2 Comments

It's not the syntax of JS literals. It's a discouraged non-standard extension explicitly forbidden in strict mode.
Right :) Never noticed that. Made me give +1 to the question. Answer fixed.
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JS treat numbers with leading zeros as octal only if they valid octal, if not then it treat it as decimal. To prevent this not use leading zeros in your source code 

console.log(010, 10, +"010") if (021 < 019) console.log('Paradox');

or use strict mode to not allow using leading zeros

'use strict' if (021 < 019) console.log('Paradox');

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1 Comment

"Uncaught SyntaxError: Decimals with leading zeros are not allowed in strict mode."

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