83 lines
2.2 KiB
Plaintext
83 lines
2.2 KiB
Plaintext
---
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description: 'Enforce using `String#startsWith` and `String#endsWith` over other equivalent methods of checking substrings.'
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---
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import Tabs from '@theme/Tabs';
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import TabItem from '@theme/TabItem';
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> 🛑 This file is source code, not the primary documentation location! 🛑
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>
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> See **https://typescript-eslint.io/rules/prefer-string-starts-ends-with** for documentation.
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There are multiple ways to verify if a string starts or ends with a specific string, such as `foo.indexOf('bar') === 0`.
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As of ES2015, the most common way in JavaScript is to use `String#startsWith` and `String#endsWith`.
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Keeping to those methods consistently helps with code readability.
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This rule reports when a string method can be replaced safely with `String#startsWith` or `String#endsWith`.
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## Examples
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<Tabs>
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<TabItem value="❌ Incorrect">
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```ts
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declare const foo: string;
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// starts with
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foo[0] === 'b';
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foo.charAt(0) === 'b';
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foo.indexOf('bar') === 0;
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foo.slice(0, 3) === 'bar';
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foo.substring(0, 3) === 'bar';
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foo.match(/^bar/) != null;
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/^bar/.test(foo);
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// ends with
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foo[foo.length - 1] === 'b';
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foo.charAt(foo.length - 1) === 'b';
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foo.lastIndexOf('bar') === foo.length - 3;
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foo.slice(-3) === 'bar';
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foo.substring(foo.length - 3) === 'bar';
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foo.match(/bar$/) != null;
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/bar$/.test(foo);
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```
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</TabItem>
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<TabItem value="✅ Correct">
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```ts
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declare const foo: string;
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// starts with
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foo.startsWith('bar');
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// ends with
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foo.endsWith('bar');
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```
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</TabItem>
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</Tabs>
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## Options
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### `allowSingleElementEquality`
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If switched to `'always'`, the rule will allow equality checks against the first or last character in a string.
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This can be preferable in projects that don't deal with special character encodings and prefer a more succinct style.
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The following code is considered incorrect by default, but is allowed with `allowSingleElementEquality: 'always'`:
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```ts option='{ "allowSingleElementEquality": "always" }' showPlaygroundButton
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declare const text: string;
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text[0] === 'a';
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text[0] === text[0].toUpperCase();
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text[0] === text[1];
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text[text.length - 1] === 'b';
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```
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## When Not To Use It
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If you don't mind which style of string checking is used, you can turn this rule off safely.
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However, keep in mind that inconsistent style can harm readability in a project.
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