Add flag to preserve dynamic import #727
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Progress toward #726 This PR adds a new `preserveDynamicImport` flag that, when enabled, changes the `imports` transform so that only static `import`s are transformed, while dynamic `import()` expressions are passed through as-is. This behavior matches what TypeScript does when handling a `.cjs` file when `module: nodenext` is specified, and ts-node expects transpilers to handle this mode, so this PR is a necessary step toward Sucrase as a ts-node plugin. This mode also seems like the best interpretation of `import()` in new code these days; if `import()` is transpiled, then it's impossible to access ESM code from CJS. My tentative plan is to change this mode to be the default behavior in an upcoming semver-major release.
Codecov Report
@@ Coverage Diff @@
## main #727 +/- ##
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Coverage 87.15% 87.16%
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Files 54 54
Lines 5708 5711 +3
Branches 1345 1346 +1
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+ Hits 4975 4978 +3
Misses 466 466
Partials 267 267
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Benchmark resultsBefore this PR: 313.1 thousand lines per second Measured change: 0.8% faster (0.18% faster to 1.88% faster) |
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Progress toward alangpierce#726 This PR adds a new `preserveDynamicImport` flag that, when enabled, changes the `imports` transform so that only static `import`s are transformed, while dynamic `import()` expressions are passed through as-is. This behavior matches what TypeScript does when handling a `.cjs` file when `module: nodenext` is specified, and ts-node expects transpilers to handle this mode, so this PR is a necessary step toward Sucrase as a ts-node plugin. This mode also seems like the best interpretation of `import()` in new code these days; if `import()` is transpiled, then it's impossible to access ESM code from CJS in Node, so leaving it alone allows a mechanism to import ESM-only libraries. My tentative plan is to change this mode to be the default behavior in an upcoming semver-major release.
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Progress toward #726
This PR adds a new
preserveDynamicImportflag that, when enabled, changes theimportstransform so that only staticimports are transformed, while dynamicimport()expressions are passed through as-is.This behavior matches what TypeScript does when handling a
.cjsfile whenmodule: nodenextis specified, and ts-node expects transpilers to handle thismode, so this PR is a necessary step toward Sucrase as a ts-node plugin.
This mode also seems like the best interpretation of
import()in new codethese days; if
import()is transpiled, then it's impossible to access ESMcode from CJS in Node, so leaving it alone allows a mechanism to import
ESM-only libraries. My tentative plan is to change this mode to be the default
behavior in an upcoming semver-major release.