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Moeki Kawakami
Moeki Kawakami

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Javascript Transpiler and ESLint are NOT Perfect

#javascript #transpiler #eslint

Summary

  • Even with a transpiler, not all ECMAScript descriptions can be converted.
  • Even with ESLint, not all ECMAScript descriptions can be linted.

Inspect!

When using ECMAScript, what does the transpiler convert and what does it parse?

  • Verify what the transpiler converts and what it ignores
  • Verify what ESLint parses and what it ignores

cc-kawakami/transpilers-and-eslint-are-not-perfect

Input for validation

  • Code samples based on ES6 to ESNext standards
  • Pick a function that you may have occasion to use 
/** * ES2015 * ------------------------------------------------ */ /** const, let */ const cst = 1 let lt = 2 console.log(cst) console.log(lt) /** Allow function */ const fn = () => { console.log('fn') } /** classes */ class Hoge { fuga () { console.log('fuga') } } /** * ES2016 * ------------------------------------------------ */ /** Array.prototype.includes */ const array = [1, 2, 3, 4, 5] console.log(array.includes(2)) /** power operator */ console.log(2**2) /** * ES2017 * ------------------------------------------------ */ /** Async functions */ async function log () { console.log('hoge') } /** Object.values() */ const obj = { hoge: 0, fuga: 1, piyo: 2 } console.log(Object.values(obj)) /** * ES2018 * ------------------------------------------------ */ /** Spread Properties */ const hoge = { fuga: 'piyo' } console.log({ .... .hoge }) /** RegExp named capture groups */ console.log(/(? <year>[0-9]{4})year/.test('2021')) /** RegExp Lookbehind Assertions */ console.log(/(? <=[0-9]+)\. [0-9]+/.test('.34')) /** * ES2019 * ------------------------------------------------ */ /** flat array methods */ console.log([[1, 2], 3, 4].flat()) /** * ES2020 * ------------------------------------------------ */ /** Optional chaining operator */ console.log(hoge?.fuga) /** * ES2021 * ------------------------------------------------ */ /** Logical assignment operators */ console.log(a ||= b) /** Numeric separators */ console.log(100_000_000) /** * ESNext * ------------------------------------------------ */ /** static class field */ class Foo { static bar = 1 } console.log(Foo.bar)
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Target transpiler and settings

No name target
1 Babel / conservative - ( > 1% in JP )
2 Babel / progressive - ( > 5% in JP )
3 esbuild / default ESNext
4 esbuild / to ES2017 ES2017
5 rollup not transpile
6 rollup-plugin-esbuild ES2017
7 Typescript / default ES3
8 Typescript / to ES2017 ES2017

Inspection results.

Descriptions not converted by ALL transpiles

  • RegExp named capture groups (ES2018)
  • RegExp Lookbehind Assertions (ES2018)
  • Flat array methods (ES2019)

Descriptions that cannot be validated with ESLint

  • flat array methods (ES2019)

Other things I noticed

  • esbuild reduces comments by default.

(extra) execution time

No name target time ( s )
1 Babel / conservative - ( > 1% in JP ) 0.63
2 Babel / progressive - ( > 5% in JP ) 0.53
3 esbuild / default ESNext 0.31
4 esbuild / to ES2017 ES2017 0.18
5 rollup not transpile 0.20
6 rollup-plugin-esbuild ES2017 0.27
7 Typescript / default ES3 1.21
8 Typescript / to ES2017 ES2017 1.36

Reference

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