Why is JSON invalid if an integer begins with a leading zero?

The UPC should be in string format. For the future you may also get other type of UPC such as GS128 or string based product identification codes. Set your DB column to be string.

One of the more confusing parts of JavaScript is that if a number starts with a 0 that isn't immediately followed by a ., it represents an octal, not a decimal.

JSON borrows from JavaScript syntax but avoids confusing features, so simply bans numbers with leading zeros (unless then are followed by a .) outright.

Even if this wasn't the case, there would be no reason to expect the 0 to still be in the number when it was parsed since 02 and 2 are just difference representations of the same number (if you force decimal).

If the leading zero is important to your data, then you probably have a string and not a number.

"UPC":"083456789012"

A product code is an identifier, not something you do maths with. It should be a string.

A leading 0 indicates an octal number in JavaScript. An octal number cannot contain an 8; therefore, that number is invalid. Moreover, JSON doesn't (officially) support octal numbers, so formally the JSON is invalid, even if the number would not contain an 8. Some parsers do support it though, which may lead to some confusion. Other parsers will recognize it as an invalid sequence and will throw an error, although the exact explanation they give may differ.

Solution: If you have a number, don't ever store it with leading zeroes. If you have a value that needs to have a leading zero, don't treat it as a number, but as a string. Store it with quotes around it.

In this case, you've got a UPC which needs to be 12 digits long and may contain leading zeroes. I think the best way to store it is as a string.

It is debatable, though. If you treat it as a barcode, seeing the leading 0 as an integral part of it, then string makes sense. Other types of barcodes can even contain alphabetic characters.

On the other hand. A UPC is a number, and the fact that it's left-padded with zeroes to 12 digits could be seen as a display property. Actually, if you left-pad it to 13 digits by adding an extra 0, you've got an EAN code, because EAN is a superset of UPC.

If you have a monetary amount, you might display it as € 7.30, while you store it as 7.3, so it could also make sense to store a product code as a number.

But that decision is up to you. I can only advice you to use a string, which is my personal preference for these codes, and if you choose a number, then you'll have to remove the 0 to make it work.

Formally, it is because JSON uses DecimalIntegerLiteral in its JSONNumber production:

JSONNumber ::
    -_opt DecimalIntegerLiteral JSONFraction_opt ExponentPart_opt

And DecimalIntegerLiteral may only start with 0 if it is 0:

DecimalIntegerLiteral ::
    0
    NonZeroDigit DecimalDigits_opt

The rationale behind is is probably:

• In the JSON Grammar - to reuse constructs from the main ECMAScript grammar.
• In the main ECMAScript grammar - to make it easier to distinguish DecimalIntegerLiteral from HexIntegerLiteral and OctalIntegerLiteral. OctalIntegerLiteral in the first place.

See this productions:

 HexIntegerLiteral ::
     0x HexDigit
     0X HexDigit
    HexIntegerLiteral HexDigit

...

OctalIntegerLiteral ::
    0 OctalDigit
    OctalIntegerLiteral OctalDigit