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Merge pull request #606 from developit/fast-rest-parameters
Fast rest parameters
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/** | ||
* @type {import('@babel/core')} | ||
*/ | ||
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/** | ||
* Transform ...rest parameters to [].slice.call(arguments,offset). | ||
* Demo: https://astexplorer.net/#/gist/70aaa0306db9a642171ef3e2f35df2e0/576c150f647e4936fa6960e0453a11cdc5d81f21 | ||
* Benchmark: https://jsperf.com/rest-arguments-babel-pr-9152/4 | ||
* @param {object} opts | ||
* @param {babel.template} opts.template | ||
* @param {babel.types} opts.types | ||
* @returns {babel.PluginObj} | ||
*/ | ||
export default function fastRestTransform({ template, types: t }) { | ||
const slice = template`var IDENT = Array.prototype.slice;`; | ||
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const VISITOR = { | ||
RestElement(path, state) { | ||
if (path.parentKey !== 'params') return; | ||
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// Create a global _slice alias | ||
let slice = state.get('slice'); | ||
if (!slice) { | ||
slice = path.scope.generateUidIdentifier('slice'); | ||
state.set('slice', slice); | ||
} | ||
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// _slice.call(arguments) or _slice.call(arguments, 1) | ||
const args = [t.identifier('arguments')]; | ||
if (path.key) args.push(t.numericLiteral(path.key)); | ||
const sliced = t.callExpression( | ||
t.memberExpression(t.clone(slice), t.identifier('call')), | ||
args, | ||
); | ||
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const ident = path.node.argument; | ||
const binding = path.scope.getBinding(ident.name); | ||
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if (binding.referencePaths.length !== 0) { | ||
// arguments access requires a non-Arrow function: | ||
const func = path.parentPath; | ||
if (t.isArrowFunctionExpression(func)) { | ||
func.arrowFunctionToExpression(); | ||
} | ||
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if (binding.constant && binding.referencePaths.length === 1) { | ||
// one usage, never assigned - replace usage inline | ||
binding.referencePaths[0].replaceWith(sliced); | ||
} else { | ||
// unknown usage, create a binding | ||
const decl = t.variableDeclaration('var', [ | ||
t.variableDeclarator(t.clone(ident), sliced), | ||
]); | ||
func.get('body').unshiftContainer('body', decl); | ||
} | ||
} | ||
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path.remove(); | ||
}, | ||
}; | ||
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return { | ||
name: 'transform-fast-rest', | ||
visitor: { | ||
Program(path, state) { | ||
const childState = new Map(); | ||
const useHelper = state.opts.helper === true; // defaults to false | ||
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if (!useHelper) { | ||
let inlineHelper; | ||
if (state.opts.literal === false) { | ||
inlineHelper = template.expression.ast`Array.prototype.slice`; | ||
} else { | ||
inlineHelper = template.expression.ast`[].slice`; | ||
} | ||
childState.set('slice', inlineHelper); | ||
} | ||
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path.traverse(VISITOR, childState); | ||
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const name = childState.get('slice'); | ||
if (name && useHelper) { | ||
const helper = slice({ IDENT: name }); | ||
t.addComment(helper.declarations[0].init, 'leading', '#__PURE__'); | ||
path.unshiftContainer('body', helper); | ||
} | ||
}, | ||
}, | ||
}; | ||
} |
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