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Warning: 08/09 is not a legal ECMA-262 octal constant

The JavaScript warning "08 (or 09) is not a legal ECMA-262 octal constant" occurs when 08 or 09 number literals are used. They can't be interpreted as an octal number.

Message

Warning: SyntaxError: 08 is not a legal ECMA-262 octal constant.

Error type

Warning. JavaScript execution won't be halted.

What went wrong?

Decimal literals can start with a zero (0) followed by another decimal digit, but If all digits after the leading 0 are smaller than 8, the number is interpreted as an octal number. Because this is not the case with 08 and 09, JavaScript warns about it.

Note that octal literals and octal escape sequences are deprecated and will present an additional deprecation warning. The standardized syntax for octal literals uses a leading zero followed by a lowercase or uppercase Latin letter "O" (0o or 0O). See the page about lexical grammar for more information.

Examples

Invalid octal numbers

08;
09;
// SyntaxError: 08 is not a legal ECMA-262 octal constant
// SyntaxError: "0"-prefixed octal literals and octal escape sequences
// are deprecated

Valid octal numbers

Use a leading zero followed by the letter "o";

0O755;
0o644;