swc-plugin: transform-module-specifiers

transform module specifier in import, export statement, such as ts ➡️ js

// Before
import { v1, f2 } from "./file3.ts";
export { v4, f5 } from "../file6.ts";

would be transformed to

// after
import { v1, f2 } from "./file3.js";
export { v4, f5 } from "../file6.js";

🛠️ Config

npm install -D swc-plugin-transform-module-specifiers

// .swcrc
{
    "plugins": [
        [
            "swc-plugin-transform-module-specifiers",
            {}
        ]
    ]
}

by default, only transform ts to js. declare extension map in plugin's config to custom transform behavior

{
    "mts": "mjs",
    "tsx": "jsx"
}

🤔 Why this plugin? or what problem it solved

if you are developing in a Node.js + TypeScript + ESM project, and transpile codes by tsc. you might come across:

import { something } from "./some-file.ts";

tsc errors that:

An import path can only end with a '.ts' extension when 'allowImportingTsExtensions' is enabled.ts(5097)

but it's wired to import "./some-file.js" in the source TS file, as the file doesn’t exist during dev time

solution for this is to change some options in tsconfig.json

{
    "compilerOptions": {
        "strict": true,
        "module": "Node16",
        "moduleResolution": "Node16",
        "noEmit": true,
        "allowImportingTsExtensions": true,
    }
}

the problem is, now you cannot produce any js code. Then turn to SWC to get these transpile work done, however SWC leaves import/export file extensions untouched.

so here comes this plugin.

References

• TypeScript imitates the host’s module resolution, but with types
• TypeScript 5.0: new mode bundler & ESM
• The TSConfig Cheat Sheet

Note

you might not need this plugin, if your project are using:

• bundler

  set tsconfig.json (refer to above cheat sheet)

  • "module": "Preserve"
  • "moduleResolution": "Bundler"

• runtime

  that able to consume TS file directly