Why does a decimal number with a 0 before it produce a SyntaxError?

Question

I was writing some Javascript when I accidentally typed 04.5 instead of 40.5. When I ran the code, it produced a SyntaxError that read Uncaught SyntaxError: Unexpected number. I tested this in Chrome, Safari, Firefox, and NodeJS, and they all produced the same error. I looked through the Javascript specifications, but I couldn't find anything on it. What is Javascript interpreting a decimal number with a 0 before it as?

04.5 // Uncaught SyntaxError: Unexpected number

Maybe its interpreting it as a boolean (false) – klediooo Aug 07 '20 at 14:48 — klediooo, Aug 07 '20 at 14:48
Maybe it is interpreted as hexadecimal? – Sergiy T. Aug 07 '20 at 14:50 — Sergiy T., Aug 07 '20 at 14:50

score 3 · Accepted Answer · answered Aug 07 '20 at 14:51

3

With a leading zero it's interpreted as octal. But it seems that octals are not liked with decimal places.

answered Aug 07 '20 at 14:51

Sascha

4,576
3
13
34

score 0 · Answer 2 · answered Aug 07 '20 at 14:48

0

It's because of the leading zeros. This works fine:

JSON.parse('[04.5]');

JSON definition says integers must not have leading zeros

EDIT To remove the leading zeros in case you have an array of numbers starting with 0 :

var str = '[04,05,06,07,08,09]';
str = str.replace(/\b0(\d)/g, "$1");
JSON.parse(str);

answered Aug 07 '20 at 14:48

Momo

66
1
9

but `04` works fine – fedesc Aug 07 '20 at 14:49
The question is about JavaScript, not JSON – Pointy Aug 07 '20 at 14:52
Right ! See the answer before ! – Momo Aug 19 '20 at 10:09

Why does a decimal number with a 0 before it produce a SyntaxError?

2 Answers2