Although not being specified in the language until ECMAScript 2021, \8 and \9 escape sequences in string literals were allowed in most JavaScript engines, and treated as "useless" escapes:

"\8" === "8"; // true
"\9" === "9"; // true

Since ECMAScript 2021, these escape sequences are specified as non-octal decimal escape sequences, retaining the same behavior.

Nevertheless, the ECMAScript specification treats \8 and \9 in string literals as a legacy feature. This syntax is optional if the ECMAScript host is not a web browser. Browsers still have to support it, but only in non-strict mode.

Regardless of your targeted environment, these escape sequences shouldn't be used when writing new code.

Rule Details

This rule disallows \8 and \9 escape sequences in string literals.

Examples of incorrect code for this rule:

::: incorrect

/*eslint no-nonoctal-decimal-escape: "error"*/

"\8";

"\9";

var foo = "w\8less";

var bar = "December 1\9";

var baz = "Don't use \8 and \9 escapes.";

var quux = "\0\8";

:::

Examples of correct code for this rule:

::: correct

/*eslint no-nonoctal-decimal-escape: "error"*/

"8";

"9";

var foo = "w8less";

var bar = "December 19";

var baz = "Don't use \\8 and \\9 escapes.";

var quux = "\0\u0038";

:::