110 lines
3.5 KiB
Markdown
110 lines
3.5 KiB
Markdown
# import/no-duplicates
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⚠️ This rule _warns_ in the following configs: ☑️ `recommended`, 🚸 `warnings`.
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🔧 This rule is automatically fixable by the [`--fix` CLI option](https://eslint.org/docs/latest/user-guide/command-line-interface#--fix).
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<!-- end auto-generated rule header -->
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Reports if a resolved path is imported more than once.
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ESLint core has a similar rule ([`no-duplicate-imports`](https://eslint.org/docs/rules/no-duplicate-imports)), but this version
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is different in two key ways:
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1. the paths in the source code don't have to exactly match, they just have to point to the same module on the filesystem. (i.e. `./foo` and `./foo.js`)
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2. this version distinguishes Flow `type` imports from standard imports. ([#334](https://github.com/import-js/eslint-plugin-import/pull/334))
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## Rule Details
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Valid:
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```js
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import SomeDefaultClass, * as names from './mod'
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// Flow `type` import from same module is fine
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import type SomeType from './mod'
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```
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...whereas here, both `./mod` imports will be reported:
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```js
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import SomeDefaultClass from './mod'
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// oops, some other import separated these lines
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import foo from './some-other-mod'
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import * as names from './mod'
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// will catch this too, assuming it is the same target module
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import { something } from './mod.js'
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```
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The motivation is that this is likely a result of two developers importing different
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names from the same module at different times (and potentially largely different
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locations in the file.) This rule brings both (or n-many) to attention.
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### Query Strings
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By default, this rule ignores query strings (i.e. paths followed by a question mark), and thus imports from `./mod?a` and `./mod?b` will be considered as duplicates. However you can use the option `considerQueryString` to handle them as different (primarily because browsers will resolve those imports differently).
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Config:
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```json
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"import/no-duplicates": ["error", {"considerQueryString": true}]
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```
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And then the following code becomes valid:
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```js
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import minifiedMod from './mod?minify'
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import noCommentsMod from './mod?comments=0'
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import originalMod from './mod'
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```
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It will still catch duplicates when using the same module and the exact same query string:
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```js
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import SomeDefaultClass from './mod?minify'
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// This is invalid, assuming `./mod` and `./mod.js` are the same target:
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import * from './mod.js?minify'
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```
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### Inline Type imports
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TypeScript 4.5 introduced a new [feature](https://devblogs.microsoft.com/typescript/announcing-typescript-4-5/#type-on-import-names) that allows mixing of named value and type imports. In order to support fixing to an inline type import when duplicate imports are detected, `prefer-inline` can be set to true.
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Config:
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```json
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"import/no-duplicates": ["error", {"prefer-inline": true}]
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```
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<!--tabs-->
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❌ Invalid `["error", {"prefer-inline": true}]`
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```js
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import { AValue, type AType } from './mama-mia'
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import type { BType } from './mama-mia'
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import { CValue } from './papa-mia'
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import type { CType } from './papa-mia'
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```
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✅ Valid with `["error", {"prefer-inline": true}]`
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```js
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import { AValue, type AType, type BType } from './mama-mia'
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import { CValue, type CType } from './papa-mia'
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```
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<!--tabs-->
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## When Not To Use It
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If the core ESLint version is good enough (i.e. you're _not_ using Flow and you _are_ using [`import/extensions`](./extensions.md)), keep it and don't use this.
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If you like to split up imports across lines or may need to import a default and a namespace,
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you may not want to enable this rule.
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