no-duplicates.md

import/no-duplicates

⚠️ This rule warns in the following configs: ☑️ recommended, 🚸 warnings.

🔧 This rule is automatically fixable by the --fix CLI option.

Reports if a resolved path is imported more than once.

ESLint core has a similar rule (no-duplicate-imports), but this version is different in two key ways:

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)
2. this version distinguishes Flow type imports from standard imports. (#334)

Rule Details

Valid:

import SomeDefaultClass, * as names from './mod'
// Flow `type` import from same module is fine
import type SomeType from './mod'

...whereas here, both ./mod imports will be reported:

import SomeDefaultClass from './mod'

// oops, some other import separated these lines
import foo from './some-other-mod'

import * as names from './mod'

// will catch this too, assuming it is the same target module
import { something } from './mod.js'

The motivation is that this is likely a result of two developers importing different names from the same module at different times (and potentially largely different locations in the file.) This rule brings both (or n-many) to attention.

Query Strings

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).

Config:

"import/no-duplicates": ["error", {"considerQueryString": true}]

And then the following code becomes valid:

import minifiedMod from './mod?minify'
import noCommentsMod from './mod?comments=0'
import originalMod from './mod'

It will still catch duplicates when using the same module and the exact same query string:

import SomeDefaultClass from './mod?minify'

// This is invalid, assuming `./mod` and `./mod.js` are the same target:
import * from './mod.js?minify'

Inline Type imports

TypeScript 4.5 introduced a new feature 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.

Config:

"import/no-duplicates": ["error", {"prefer-inline": true}]

❌ Invalid ["error", {"prefer-inline": true}]

import { AValue, type AType } from './mama-mia'
import type { BType } from './mama-mia'

import { CValue } from './papa-mia'
import type { CType } from './papa-mia'

✅ Valid with ["error", {"prefer-inline": true}]

import { AValue, type AType, type BType } from './mama-mia'

import { CValue, type CType } from './papa-mia'

When Not To Use It

If the core ESLint version is good enough (i.e. you're not using Flow and you are using import/extensions), keep it and don't use this.

If you like to split up imports across lines or may need to import a default and a namespace, you may not want to enable this rule.