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# Prefer `export…from` when re-exporting | ||
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When re-exporting from a module, it's unnecessary to import and then export. It can be done in a single `export…from` declaration. | ||
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This rule is fixable. | ||
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## Fail | ||
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```js | ||
import defaultExport from './foo.js'; | ||
export default defaultExport; | ||
``` | ||
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```js | ||
import {named} from './foo.js'; | ||
export {named}; | ||
``` | ||
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```js | ||
import * as namespace from './foo.js'; | ||
export {namespace}; | ||
``` | ||
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```js | ||
import defaultExport, {named} from './foo.js'; | ||
export default defaultExport; | ||
export { | ||
defaultExport as renamedDefault, | ||
named, | ||
named as renamedNamed, | ||
}; | ||
``` | ||
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## Pass | ||
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```js | ||
export {default} from './foo.js'; | ||
``` | ||
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```js | ||
export {named} from './foo.js'; | ||
``` | ||
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```js | ||
export * as namespace from './foo.js'; | ||
``` | ||
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```js | ||
export { | ||
default, | ||
default as renamedDefault, | ||
named, | ||
named as renamedNamed, | ||
} from './foo.js'; | ||
``` | ||
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```js | ||
// There is no substitution | ||
import * as namespace from './foo.js'; | ||
export default namespace; | ||
``` |
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Original file line number | Diff line number | Diff line change |
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'use strict'; | ||
const { | ||
isCommaToken, | ||
isOpeningBraceToken, | ||
isClosingBraceToken, | ||
} = require('eslint-utils'); | ||
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const MESSAGE_ID = 'prefer-export-from'; | ||
const messages = { | ||
[MESSAGE_ID]: 'Use `export…from` to re-export `{{exported}}`.', | ||
}; | ||
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function * removeSpecifier(node, fixer, sourceCode) { | ||
const {parent} = node; | ||
const {specifiers} = parent; | ||
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if (specifiers.length === 1) { | ||
yield * removeImportOrExport(parent, fixer, sourceCode); | ||
return; | ||
} | ||
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switch (node.type) { | ||
case 'ImportSpecifier': { | ||
const hasOtherSpecifiers = specifiers.some(specifier => specifier !== node && specifier.type === node.type); | ||
if (!hasOtherSpecifiers) { | ||
const closingBraceToken = sourceCode.getTokenAfter(node, isClosingBraceToken); | ||
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// If there are other specifiers, they have to be the default import specifier | ||
// And the default import has to write before the named import specifiers | ||
// So there must be a comma before | ||
const commaToken = sourceCode.getTokenBefore(node, isCommaToken); | ||
yield fixer.replaceTextRange([commaToken.range[0], closingBraceToken.range[1]], ''); | ||
return; | ||
} | ||
// Fallthrough | ||
} | ||
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case 'ExportSpecifier': | ||
case 'ImportNamespaceSpecifier': | ||
case 'ImportDefaultSpecifier': { | ||
yield fixer.remove(node); | ||
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const tokenAfter = sourceCode.getTokenAfter(node); | ||
if (isCommaToken(tokenAfter)) { | ||
yield fixer.remove(tokenAfter); | ||
} | ||
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break; | ||
} | ||
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// No default | ||
} | ||
} | ||
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function * removeImportOrExport(node, fixer, sourceCode) { | ||
switch (node.type) { | ||
case 'ImportSpecifier': | ||
case 'ExportSpecifier': | ||
case 'ImportDefaultSpecifier': | ||
case 'ImportNamespaceSpecifier': { | ||
yield * removeSpecifier(node, fixer, sourceCode); | ||
return; | ||
} | ||
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case 'ImportDeclaration': | ||
case 'ExportDefaultDeclaration': | ||
case 'ExportNamedDeclaration': { | ||
yield fixer.remove(node); | ||
} | ||
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// No default | ||
} | ||
} | ||
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function fix({ | ||
context, | ||
imported, | ||
exported, | ||
exportDeclarations, | ||
program, | ||
}) { | ||
const sourceCode = context.getSourceCode(); | ||
const sourceNode = imported.declaration.source; | ||
const sourceValue = sourceNode.value; | ||
const sourceText = sourceCode.getText(sourceNode); | ||
const exportDeclaration = exportDeclarations.find(({source}) => source.value === sourceValue); | ||
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/** @param {import('eslint').Rule.RuleFixer} fixer */ | ||
return function * (fixer) { | ||
if (imported.name === '*') { | ||
yield fixer.insertTextAfter( | ||
program, | ||
`\nexport * as ${exported.name} from ${sourceText};`, | ||
); | ||
} else { | ||
const specifier = exported.name === imported.name | ||
? exported.name | ||
: `${imported.name} as ${exported.name}`; | ||
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if (exportDeclaration) { | ||
const lastSpecifier = exportDeclaration.specifiers[exportDeclaration.specifiers.length - 1]; | ||
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// `export {} from 'foo';` | ||
if (lastSpecifier) { | ||
yield fixer.insertTextAfter(lastSpecifier, `, ${specifier}`); | ||
} else { | ||
const openingBraceToken = sourceCode.getFirstToken(exportDeclaration, isOpeningBraceToken); | ||
yield fixer.insertTextAfter(openingBraceToken, specifier); | ||
} | ||
} else { | ||
yield fixer.insertTextAfter( | ||
program, | ||
`\nexport {${specifier}} from ${sourceText};`, | ||
); | ||
} | ||
} | ||
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if (imported.variable.references.length === 1) { | ||
yield * removeImportOrExport(imported.node, fixer, sourceCode); | ||
} | ||
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yield * removeImportOrExport(exported.node, fixer, sourceCode); | ||
}; | ||
} | ||
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function getImportedName(specifier) { | ||
switch (specifier.type) { | ||
case 'ImportDefaultSpecifier': | ||
return 'default'; | ||
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case 'ImportSpecifier': | ||
return specifier.imported.name; | ||
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case 'ImportNamespaceSpecifier': | ||
return '*'; | ||
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// No default | ||
} | ||
} | ||
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function getExported(identifier, context) { | ||
const {parent} = identifier; | ||
switch (parent.type) { | ||
case 'ExportDefaultDeclaration': | ||
return { | ||
node: parent, | ||
name: 'default', | ||
}; | ||
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case 'ExportSpecifier': | ||
return { | ||
node: parent, | ||
name: parent.exported.name, | ||
}; | ||
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case 'VariableDeclarator': { | ||
if ( | ||
parent.init === identifier | ||
&& parent.id.type === 'Identifier' | ||
&& parent.parent.type === 'VariableDeclaration' | ||
&& parent.parent.kind === 'const' | ||
&& parent.parent.declarations.length === 1 | ||
&& parent.parent.declarations[0] === parent | ||
&& parent.parent.parent.type === 'ExportNamedDeclaration' | ||
&& isVariableUnused(parent, context) | ||
) { | ||
return { | ||
node: parent.parent.parent, | ||
name: parent.id.name, | ||
}; | ||
} | ||
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break; | ||
} | ||
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// No default | ||
} | ||
} | ||
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function isVariableUnused(node, context) { | ||
const variables = context.getDeclaredVariables(node); | ||
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/* istanbul ignore next */ | ||
if (variables.length !== 1) { | ||
return false; | ||
} | ||
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const [{identifiers, references}] = variables; | ||
return identifiers.length === 1 | ||
&& identifiers[0] === node.id | ||
&& references.length === 1 | ||
&& references[0].identifier === node.id; | ||
} | ||
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function * getProblems({ | ||
context, | ||
variable, | ||
program, | ||
exportDeclarations, | ||
}) { | ||
const {identifiers, references} = variable; | ||
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if (identifiers.length !== 1 || references.length === 0) { | ||
return; | ||
} | ||
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const specifier = identifiers[0].parent; | ||
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const imported = { | ||
name: getImportedName(specifier), | ||
node: specifier, | ||
declaration: specifier.parent, | ||
variable, | ||
}; | ||
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for (const {identifier} of references) { | ||
const exported = getExported(identifier, context); | ||
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if (!exported) { | ||
continue; | ||
} | ||
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/* | ||
There is no substitution for: | ||
```js | ||
import * as foo from 'foo'; | ||
export default foo; | ||
``` | ||
*/ | ||
if (imported.name === '*' && exported.name === 'default') { | ||
return; | ||
} | ||
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yield { | ||
node: exported.node, | ||
messageId: MESSAGE_ID, | ||
data: { | ||
exported: exported.name, | ||
}, | ||
fix: fix({ | ||
context, | ||
imported, | ||
exported, | ||
exportDeclarations, | ||
program, | ||
}), | ||
}; | ||
} | ||
} | ||
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/** @param {import('eslint').Rule.RuleContext} context */ | ||
function create(context) { | ||
const variables = []; | ||
const exportDeclarations = []; | ||
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return { | ||
'ImportDeclaration[specifiers.length>0]'(node) { | ||
variables.push(...context.getDeclaredVariables(node)); | ||
}, | ||
// `ExportAllDeclaration` and `ExportDefaultDeclaration` can't be reused | ||
'ExportNamedDeclaration[source.type="Literal"]'(node) { | ||
exportDeclarations.push(node); | ||
}, | ||
* 'Program:exit'(program) { | ||
for (const variable of variables) { | ||
yield * getProblems({ | ||
context, | ||
variable, | ||
exportDeclarations, | ||
program, | ||
}); | ||
} | ||
}, | ||
}; | ||
} | ||
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module.exports = { | ||
create, | ||
meta: { | ||
type: 'suggestion', | ||
docs: { | ||
description: 'Prefer `export…from` when re-exporting.', | ||
}, | ||
fixable: 'code', | ||
messages, | ||
}, | ||
}; |
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