Why Does `040` Equal `32` in JavaScript?

Leading zero numbers in JavaScript

Question:

Why is 40 represented as 32 in JavaScript?

var num = 040;
console.log(num); // 32

Answer:

In JavaScript, the number will be treated as octal (base 8) due to the leading 0, just like the leading 0x will make it Same as hexadecimal (base 16). This behavior has a long history, but in modern JavaScript, octal numbers are no longer written in this form. In modern JavaScript using strict mode, the "traditional" octal format is a syntax error; octal numbers should be written with the 0o prefix.

History:

In the early days (in the original Netscape language and the first and second ECMAScript specifications), leading 0's on numeric literals formally represented octal ( base 8), just like the leading 0x indicates hexadecimal (base 16):

OctalIntegerLiteral ::
    0 OctalDigit
    OctalIntegerLiteral OctalDigit

For example, 10, 012, and 0xA are all ways of writing the decimal number 10. This is consistent with some other languages ​​(such as C, C++, and Java) whose syntax is similar to JavaScript, but is very confusing.

Starting in ECMAScript 3, this form of octal literals was demoted to an optional extension, and decimal integer literals were changed so that they cannot have leading zeros (unless the implementation includes the extension):

DecimalIntegerLiteral ::
    0
    NonZeroDigit DecimalDigits(opt)

But in strict mode, ECMAScript 5 prohibits this:

When dealing with strict mode code (see 10.1.1), a conforming implementation SHOULD not extend the syntax of NumericLiteral to include OctalIntegerLiteral as described in B.1.1.

ECMAScript 6 (ECMAScript 2015) introduced BinaryIntegerLiteral and OctalIntegerLiteral, so now we have more coherent literals:

• BinaryIntegerLiteral, prefixed with 0b or 0B.
• OctalIntegerLiteral, with prefix 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:

So if you want to parse octal numbers, you can use 0o or 0O prefix (not supported by older browsers), or use parseInt.

If you want to ensure your number will be parsed as base 10, remove leading zeros, or use parseInt.

Example:

• 010

  • in strict mode (requires ECMAScript 5), it is a syntax error.
  • In non-strict mode, it may be a syntax error or return 8 (implementation dependent).

• 0o10, 0O10

  • Before ECMAScript 6, they were syntax errors.
  • In ECMAScript 6, they return 8.

• parseInt('010', 8)

  • It returns 8.

• parseInt('010', 10)

  • It returns 10.

If you are interested, you can find the current latest specifications here and historical versions here.

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