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Support bare internal mappings #30

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Support bare internal mappings #30

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5 changes: 4 additions & 1 deletion README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -54,13 +54,16 @@ Here’s a complete `package.json` example, for a hypothetical module named `@mo
".": "./src/moment.mjs",
"./": "./src/util/",
"./timezones/": "./data/timezones/",
"./timezones/utc": "./data/timezones/utc/index.mjs"
"./timezones/utc": "./data/timezones/utc/index.mjs",
"localdep": "./third-party/localdep.mjs"
}
}
```

Within the `"exports"` object, the key string after the `'.'` is concatenated on the end of the name field, e.g. `import utc from '@momentjs/moment/timezones/utc'` is formed from `'@momentjs/moment'` + `'/timezones/utc'`. Note that this is string manipulation, not a file path: `"./timezones/utc"` is allowed, but just `"timezones/utc"` is not. The `.` is a placeholder representing the package name. The main entrypoint is therefore the dot string, `".": "./src/moment.mjs"`.

For keys not beginning in `.`, these are mappings which apply internally to any imports made from within the package itself. So `src/moment.mjs` could contain for example `import dep from 'localdep'` which would load from a package-relative location. For example one could define `"moment": "./src/moment.mjs"` allowing the package to refer to itself by name as well in this way. These mappings cannot be used at all from outside of the package.
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This seems to preclude being able to remap a file to a node_module. Also what about paths beginning in /?

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Because this mapping operation happens early, it could even be a node_modules package lookup actually.

We could allow arbitrary paths or not. Thoughts?

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I actually think this should forbid paths that don't start with ..

npm 6.9 has an alias feature that can be used to remap bare imports - I don't think there's any benefit in us duplicating it.


Keys that end in slashes can map to folder roots, following the [pattern in the browser import maps proposal](https://github.com/WICG/import-maps#packages-via-trailing-slashes): `"./timezones/": "./data/timezones/"` would allow `import pdt from "@momentjs/moment/timezones/pdt.mjs"` to import `./data/timezones/pdt.mjs`.

- Using `"./"` maps the root, so `"./": "./src/util/"` would allow `import tick from "@momentjs/moment/tick.mjs"` to import `./src/util/tick.mjs`.
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