SyntaxError: octal escape sequences can't be used in untagged template literals or in strict mode code

The JavaScript exception "octal escape sequences can't be used in untagged template literals or in strict mode code" occurs when octal escape sequences are used in strict mode string literals or untagged template literals.

Message

SyntaxError: Octal escape sequences are not allowed in strict mode. (V8-based)
SyntaxError: \8 and \9 are not allowed in strict mode. (V8-based)
SyntaxError: Octal escape sequences are not allowed in template strings. (V8-based)
SyntaxError: \8 and \9 are not allowed in template strings. (V8-based)
SyntaxError: octal escape sequences can't be used in untagged template literals or in strict mode code (Firefox)
SyntaxError: the escapes \8 and \9 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

What went wrong?

The string escape sequence of the form \ followed by any number of digits, except a single 0, is deprecated. If you want to represent a character by its code point value, you should use the \x or \u escape sequence instead, such as \x01 or \u0001 instead of \1.

Untagged template literals are never allowed to contain octal escape sequences, whether in strict mode or not. However, tagged template literals can contain any form of escape sequence, and will cause the template array received by the tag function to contain undefined.

Examples

Octal escape sequences

js
"use strict";

"\251";

// SyntaxError: octal escape sequences can't be used in untagged template literals or in strict mode code

Valid octal numbers

For octal escape sequences, you can use hexadecimal escape sequences instead:

js
"\xA9";

If you want to represent some source text literally without interpreting any escape sequence, use String.raw:

js
String.raw`\251`; // A string containing four characters

See also