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a4ae4505f1
Compress `[].concat(a).concat(b)` into `[].concat(a, b)`. **References** - [Spec of `Array::concat`](https://tc39.es/ecma262/multipage/indexed-collections.html#sec-array.prototype.concat) --- ### The new assumption I added a new assumption description in `crates/oxc_minifier/README.md`: "Errors thrown when creating a String or an Array that exceeds the maximum length can disappear or moved". This is an assumption held by other minifiers. Without this assumption, we have to treat `+` and array creation/update to have a sideeffect and that limits the minification. For input: ```js [...Array(Number(2n ** 32n - 1n)),""] " ".repeat(Number(2n ** 29n - 24n - 1n)) + ' ' export {} ``` (Note that `2 ** 32 - 1` is the max array length and `2 ** 29 - 24` is the max string length on V8.) This code errors with too long array and too long string on V8 based runtimes. terser outputs: ```js Array(Number(2n**32n-1n))," ".repeat(Number(536870887n));export{}; ``` No error will happen with this code. SWC outputs: ```js [...Array(Number(2n**32n-1n))]," ".repeat(Number(2n**29n-24n-1n));export{}; ``` No error will happen with this code. [playground](https://play.swc.rs/?version=1.10.7&code=H4sIAAAAAAAAA4vW09NzLCpKrNTwK81NSi3SMMpT0NJSMAZSugqGeZqaOkpKsbxcSgpKekWpBamJJajqjCxB6oxMYKoVtBXUFdR5uVIrCvKLShSqa3m5AGRqy%2FBiAAAA&config=H4sIAAAAAAAAA32UO3LjMAyG%2B5zCozrFjostcoDtcgYOTYIyvXxoCNCxJuO7L0RJtjeG3En48AMkAOL7bbfrTmi6j903f%2FLPoAtCuf2zBcdE%2BsKWDkzUaIofqHtf6Qkn5HRAaKbrTDrSpQdqKtz%2F2u8XRRdyRlgViy365N34mNPkOBRAfLCxlUPWCInwf%2F3CSv6aAJX6aD%2FkHECnF0RpVD4R9FCkwCaHoAcEddZFiDKdVBePWUoxwUpg1VDyIPJkPfmcOOcztaCtMtmCgHwBQ%2F4MkoxzsSwhX0%2B4T8MWDrXvW59%2FqOGsQ9Uk5IRLawmfVoh6zB5JuZqkEs5wowYzXIr7U%2BmdKkC1pGfdKfu00ZO%2FAFyBoBGTjiDFbR6O52lL7V4qfXI8sjQKnOdbumWCnouqvHdCZafKQCEvdbOArQamyhrpOAveKB96Cwqc41kRQuOXJ3OUktI4QHYC4P5qJ03VDNTtFW7w6UG8wH%2F4liQP2OIRNR23KY7xkMOLBBHomO0LB24F5W1ceEtchm1ekwUeDbCiS8UGnpcAPwDKKrR9%2BTQb%2FDw4oupDPtzXxOJwve3hqFN%2Ff%2B%2FzKn5bHLqYbW1wWfJTf%2BfV%2FLu7O61beD1B5%2FFzFbac13%2FKOhgeLwYAAA%3D%3D) esbuild outputs: ```js [...Array(Number(2n**32n-1n))]," ".repeat(Number(2n**29n-24n-1n))+"";export{}; ``` No error will happen with this code. [esbuild try](https://esbuild.github.io/try/#dAAwLjI0LjIALS1taW5pZnkAWy4uLkFycmF5KE51bWJlcigybiAqKiAzMm4gLSAxbikpLCIiXQoiICIucmVwZWF0KE51bWJlcigybiAqKiAyOW4gLSAyNG4gLSAxbikpICsgJyAnCmV4cG9ydCB7fQ) OXC outputs: ```js [...Array(Number(2n**32n-1n))]," ".repeat(Number(2n**29n-24n-1n))+" ";export{}; ``` The array error won't happen and the String error will happen with this code. [playground](https://playground.oxc.rs/#eNpVT71OwzAQfhXrlv4QIhpgIBtLR8SOGZxwCUH22To7baMo746dNEVMd5/u+7sRaijhI8/zV2Y1bN96UyFvCxL7vXiM414caLfLJEj4lCRBSMgZHarwn1u8JG7xtCrEndiIjSS8OMtBjJMkyMBCOQL3lIYfKKgLlIF7zEB3FKBslPYR+No6XC9+MJXVKwqsyDeWzZU8ZeAUe+TZ0vZc47HTSMpEAwjoQ/7jY7JjjKQTvitG8n/ilDuL437zXtyC4hZjKUBfPByeITJq+4UtpvoRmI66plu4tTUpIRovNc/fXcx2qr69YRS1+opmJwps9VHbc9KfkCvr43npNU2/v/WIsw==) |
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| .. | ||
| examples | ||
| src | ||
| tests | ||
| Cargo.toml | ||
| CHANGELOG.md | ||
| README.md | ||
Minifier
A JavaScript minifier has three components:
- printer
- mangler
- compressor
Mangler
The mangler implementation is part of the SymbolTable residing in oxc_semantic.
It is responsible for shortening variables. Its algorithm should be gzip friendly.
The printer is also responsible for printing out the shortened variable names.
Compressor
The compressor is responsible for rewriting statements and expressions for minimal text output. Terser is a good place to start for learning the fundamentals.
Assumptions
- Properties of the global object defined in the ECMAScript spec behaves the same as in the spec
- Examples of properties:
Infinity,parseInt,Object,Promise.resolve - Examples that breaks this assumption:
globalThis.Object = class MyObject {}
- Examples of properties:
document.allis not used or behaves as a normal object- Examples that breaks this assumption:
console.log(typeof document.all === 'undefined')
- Examples that breaks this assumption:
- TDZ violation does not happen
- Examples that breaks this assumption:
(() => { console.log(v); let v; })()
- Examples that breaks this assumption:
withstatement is not used- Examples that breaks this assumption:
with (Math) { console.log(PI); }
- Examples that breaks this assumption:
- Errors thrown when creating a String or an Array that exceeds the maximum length can disappear or moved
- Examples that breaks this assumption:
try { new Array(Number(2n**53n)) } catch { console.log('log') }
- Examples that breaks this assumption:
Terser Tests
The fixtures are copied from https://github.com/terser/terser/tree/v5.9.0/test/compress