fix(terser): replace __filename with import.meta.url

[jonkoops]

Rollup Plugin Name: terser

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

If yes, then include "BREAKING CHANGES:" in the first commit message body, followed by a description of what is breaking.

List any relevant issue numbers:

• Closes [@rollup/plugin-terser] esm module uses __filename #1366
• Supersedes fix(terser): __filename not defined #1367

Description

Because __filename does not exist in the ES modules this plugin errors out when used in a Rollup config written in the same format. This PR replaces the __filename constant with import.meta.url instead, which is available in ES modules.

When building the CommonJS version import.meta.url is replaced with __dirname at compile time, making sure that it remains compatible.

[jonkoops]

Here is an example of the transpiled CommonJS code that initializes the worker. I believe this is enough to keep backwards compatibility:

const workerPool = new WorkerPool({
    filePath: url.fileURLToPath((typeof document === 'undefined' ? new (require('u' + 'rl').URL)('file:' + __filename).href : (document.currentScript && document.currentScript.src || new URL('index.js', document.baseURI).href))),
    maxWorkers: options.maxWorkers
});

[jonkoops]

Had to set this flag, otherwise the TypeScript compiler will start complaining about using import.meta.url in something that compiles to a CommonJS target. Which of course is a non-issue as Rollup is used to compile the code, not the TypeScript compiler.

That said, should this not be the default? Then opting back into CommonJS where necessary (not even sure if that is needed).

[jonkoops]

Closing this PR in favor of #1367, which has now been updated with a similar implementation.