es6: prefer String.startsWith() and String.includes()

[NovemLinguae]

String.startsWith() and String.includes() were introduced in ES6 and are great. Very readable.

• Warn or error on String.indexOf(a) !== -1, suggest String.includes() instead
  • Autofix
• Warn or error on String.indexOf(a) === 0, suggest String.startsWith() instead
  • Autofix

Source code from another eslint plugin with these rules here and here (MIT license)

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edit: Array.indexOf() !== -1 is also a very common pattern. Might also want to tackle it at the same time.

• Warn or error on Array.indexOf(a) !== -1, suggest Array.includes() instead
  • Autofix

Careful of this pattern when autofixing: (badIndex = params.tags.indexOf('Bad GIF')) !== -1. That should not be autofixed due to the variable depending on getting the indexOf's value.

[edg2s]

We can use the already present unicorn plugin for startsWith:

• https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-string-starts-ends-with.md
  although it only will detect a regex, as indexOf could be form an Array, and we can't tell the difference without typescript: prefer-starts-ends-with: Support string.indexOf('') === 0  sindresorhus/eslint-plugin-unicorn#712

[edg2s]

Alternatively the rules could be applied to ES2016 rules only, where Array.indexOf can also be replaced by Array.includes, and so the type wouldn't matter

[NovemLinguae]

543 hits in CodeSearch for !== -1, and 200 for === 0. Lots of good potential autofixing here.

edit: Actually, some of those may be Array.indexOf().

[NovemLinguae]

Looks like unicorn/prefer-starts-ends-with only catches RegEx, not indexOf.

Once #599 is merged, though, we can turn on unicorn/prefer-includes, and then let LibUp run autofixes for it. It looks like that one will convert all Array.indexOf to Array.includes for us.

Example autofixes in Twinkle.

[NovemLinguae]

Note that unicorn/prefer-includes autofixes are buggy sometimes, so be careful with it. Example: https://en.wikipedia.org/wiki/MediaWiki_talk:Gadget-popups.js#str.includes_is_not_a_function