The JavaScript strict mode-only exception "0-prefixed octal literals and octal escape sequences are deprecated; for octal literals use the "0o" prefix instead" occurs when deprecated octal literals and octal escape sequences are used.
Message
SyntaxError: Octal literals are not allowed in strict mode. (V8-based)
SyntaxError: "0"-prefixed octal literals are deprecated; use the "0o" prefix instead (Firefox)
SyntaxError: Decimal integer literals with a leading zero are forbidden in strict mode (Safari)
SyntaxError: Octal escape sequences are not allowed in strict mode. (V8-based)
SyntaxError: octal escape sequences can't be used in untagged template literals or in strict mode code (Firefox)
SyntaxError: The only valid numeric escape in strict mode is '\0' (Safari)
Error type
SyntaxError in strict mode only.
What went wrong?
Octal literals and octal escape sequences are deprecated and will throw a
SyntaxError in strict mode. The
standardized syntax uses a leading zero followed by a lowercase or uppercase Latin
letter "O" (0o or 0O).
Examples
"0"-prefixed octal literals
"use strict";
03;
// SyntaxError: "0"-prefixed octal literals are deprecated; use the "0o" prefix instead
Octal escape sequences
"use strict";
"\251";
// SyntaxError: octal escape sequences can't be used in untagged template literals or in strict mode code
Valid octal numbers
Use a leading zero followed by the letter "o" or "O":
0o3;
For octal escape sequences, you can use hexadecimal escape sequences instead:
"\xA9";