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[cli] Avoid SourceMapGenerator for simple map concatenation #14479

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merged 2 commits into from Apr 21, 2022

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@jridgewell jridgewell commented Apr 20, 2022

Q                       A
Fixed Issues?
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass? Yes
Documentation PR Link
Any Dependency Changes? Yes, updates @jridgewell/trace-mapping and removes ancient source-map
License MIT

This removes the last use of source-map package from babel-cli. Instead, we use a SectionedSourceMap, which is purpose built for simple file concatenation like this.

This should be considerably faster to generate sourcemaps now. If sourcemaps are not needed, it's essentially free. And as an added benefit, this gives us proper names support:

--- a	2022-04-20 11:01:49.000000000 -0400
+++ b	2022-04-20 11:01:55.000000000 -0400
@@ -1,12 +1,17 @@
 {
   "version": 3,
   "file": "script2.js",
-  "names": [],
+  "names": [ "arr", "map", "x", "MULTIPLIER" ],
   "sources": [
     "script.js"
   ],
   "sourcesContent": [
     "arr.map(x => x * MULTIPLIER);"
   ],
-  "mappings": ";;AAAA,GAAG,CAAC,GAAJ,CAAQ,UAAA,CAAC;AAAA,SAAI,CAAC,GAAG,UAAR;AAAA,CAAT"
+  "mappings": ";;AAAAA,GAAG,CAACC,GAAJ,CAAQ,UAAAC,CAAC;AAAA,SAAIA,CAAC,GAAGC,UAAR;AAAA,CAAT"
 }

(That fifth letter that appears in some mappings segments is the names pointer)

This removes the last use of `source-map` package from babel-cli. Instead, we use a [SectionedSourceMap](https://github.com/jridgewell/trace-mapping/blob/8db5df070b15e2a669277b710a9d69ad0676524d/src/types.ts#L28-L32), which is purpose built for simple file concatenation like this.

This should be considerably faster to generate sourcemaps now. And, if a sourcemaps are not needed, it's essentially free. As an added benefit, this gives us proper `names` support:

```diff
--- a	2022-04-20 11:01:49.000000000 -0400
+++ b	2022-04-20 11:01:55.000000000 -0400
@@ -1,12 +1,17 @@
 {
   "version": 3,
   "file": "script2.js",
-  "names": [],
+  "names": [ "arr", "map", "x", "MULTIPLIER" ],
   "sources": [
     "script.js"
   ],
   "sourcesContent": [
     "arr.map(x => x * MULTIPLIER);"
   ],
-  "mappings": ";;AAAA,GAAG,CAAC,GAAJ,CAAQ,UAAA,CAAC;AAAA,SAAI,CAAC,GAAG,UAAR;AAAA,CAAT"
+  "mappings": ";;AAAAA,GAAG,CAACC,GAAJ,CAAQ,UAAAC,CAAC;AAAA,SAAIA,CAAC,GAAGC,UAAR;AAAA,CAAT"
 }
```
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@babel-bot babel-bot commented Apr 20, 2022

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/51708/

@JLHwung JLHwung added PR: Bug Fix 🐛 area: sourcemaps labels Apr 20, 2022
function countNewlines(code: string): number {
let count = 0;
let index = -1;
while ((index = code.indexOf("\n", index + 1)) !== -1) {
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@nicolo-ribaudo nicolo-ribaudo Apr 20, 2022

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I think we need to handle all the possible newlines. Even if we always print \n, multiline template literals may still contain different line terminators.

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@jridgewell jridgewell Apr 20, 2022

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This is just a faster version of the code.split("\n").length that was there before. We can add more newline support, could we wait until a followup?

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@nicolo-ribaudo nicolo-ribaudo Apr 21, 2022

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Oh ok sure, I didn't realize it was already like this 👍 (and no one ever complained, so it's super low priority).

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@nicolo-ribaudo nicolo-ribaudo left a comment

Thanks!

@nicolo-ribaudo nicolo-ribaudo changed the title babel-cli: avoid SourceMapGenerator for simple map concatenation [cli] Avoid SourceMapGenerator for simple map concatenation Apr 21, 2022
@nicolo-ribaudo nicolo-ribaudo merged commit f51fc61 into babel:main Apr 21, 2022
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@jridgewell jridgewell deleted the cli-anymap branch May 11, 2022
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4 participants