Fix: autofix shouldn't produce template literals with \8 or \9

[mdjermanovic]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[X] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

This PR fixes auto-fixing in two rules:

• prefer-template
• quotes (with option "backtick")

The example is with prefer-template.

Tell us about your environment

• ESLint Version: v7.10.0
• Node Version: v12.18.4
• npm Version: v6.14.6

What parser (default, @babel/eslint-parser, @typescript-eslint/parser, etc.) are you using?

default

Please show your full configuration:

Configuration

module.exports = {
  parserOptions: {
    ecmaVersion: 2015
  }
};

What did you do? Please include the actual source code causing the issue.

Online Demo

/* eslint prefer-template: "error" */

foo + "\8"

What did you expect to happen?

1 error without autofix, similar to how it already works with octal escape sequences.

8 and 9 were never valid escape sequences or valid legacy octal escape sequences per the specification. However, they were not an explicitly disallowed language extension.

Since tc39/ecma262#2054, they are defined as NonOctalDecimalEscapeSequence and explicitly disallowed in untagged template literals:

TemplateCharacter must not be extended to include LegacyOctalEscapeSequence or NonOctalDecimalEscapeSequence as defined in B.1.2.

NonOctalDecimalEscapeSequence :: one of
89

What actually happened? Please include the actual, raw output from ESLint.

1 error and autofix to:

/* eslint prefer-template: "error" */

`${foo  }\8`

Error after the autofix:

  3:11  error  Parsing error: Invalid escape sequence in template string

(this is a parsing error as of Acorn v7.1.1)

What changes did you make? (Give an overview)

Fixed the pattern used by astUtils.hasOctalEscapeSequence to include \8 and \9.

This will prevent invalid autofixes in prefer-template and quotes rules in cases where the original string literal had at least one of \8 or \9. The rules will still report errors, but without fixing.

Also simplified the start of the pattern.

Is there anything you'd like reviewers to focus on?

[btmills]

LGTM. Thanks for figuring out how to handle these safely.

[btmills]

I like how you simplified these two helpers down to a single recursive function. It's easier to follow.