`tape` runner does not support ESM files

[donmccurdy]

Reading #414 — which I realize is about writing Tape with ES modules, not testing ES modules — I found this suggestion:

#414 (comment)

In case someone stumbles upon this, passing --experimental-modules to tape works fine on windows (nix not available or I'd test). Even in a context such as this: npx tape --experimental-modules test/**/.mjs.

Testing now, in node.js v12.9.1 and macOS, I'm getting a silent failure when trying to use that. For example:

import test from 'tape';
// import * as test from 'tape';
// import { test } from 'tape';

test('test', function (t) {
  t.equal( 'test', 'test', 'it is a test' );
  t.end();
});

tape --experimental-modules my-test.js

Should this be supported? It may be worth noting that I also get no output running tape -v or tape -h. I'm not sure if that indicates a problem with my environment.

[ljharb]

You no longer need the experimental modules flag on node v13.2+, fwiw.

Separately, on node 12, prior to v12.16, the experimental modules implementation is incomplete and/or broken. So, if you want to use ESM, you must use node ^12.16 || >= 13.2, with or without the flag. It's not possible to have node v12.9 work properly with ESM.

[donmccurdy]

Thanks — updating node.js would be ok for me, but I seem to be getting the same result on v13.12.0. Are there any examples of doing this correctly?

Continuing to test the example above:

• tape my-test.js --> Cannot use import statement outside a module
• tape --experimental-modules my-test.js --> silent failure
• tape my-test.mjs --> Must use import to load ES Module

[ljharb]

Ah, here we go :-) ESM must be written in an .mjs file (unless the project's package.json has "type": "module", which I'd discourage. However, tape currently uses require, which can only load CJS files.

As such, you'd currently have to pass a CJS file to the tape runner that used import() to get at the ESM files, and then awaited that promise, which is admittedly super awkward.

You can, of course, run all tape tests with node - ie, node my-test.mjs should work just fine, but only for a single file.

I'm not sure how it'd be possible for the tape runner to both support pre-ESM nodes, and also conditionally use import() to load test files, which is a syntax error in older nodes, but I'm very open to PRs if we can figure out a way to do it.

[donmccurdy]

The ES module imported by my actual project was in a dependency, so changing the file extensions or import syntax would have been tricky for sure. Instead, I was able to get this working well with the esm package:

require = require('esm')(module);

const test = require('tape');
const foo = require('foo'); // 'foo' package contains references to ES modules

test('test', function(t) {
  // ...
});

With that, the usual syntax works fine:

tape my-test.js

It seems to have some limitations with .mjs files and type: "module", but I'd rather avoid both of those things, so this is fine.

Thanks for looking into this! Feel free to close the issue; I don't think any fixes are necessary here except perhaps a tip in the docs?

[ljharb]

Nah, the esm package shouldn't be needed in any ESM-supporting node, and without .mjs or "type": "module", it's simply not ESM, so I'll keep this open to track some kind of effort for the tape runner. Thanks for reporting!

[Raynos]

Should we add a "known to be working example" to the README for ESM ?

Something like node test.mjs will always work with the correct import statement.

We could also open a PR for ./bin/tape.js that if a file ends with .mjs use dynamic import() instead of require and then add tape test/*.mjs to the README

[ljharb]

@Raynos there's no way to conditionally use import() and still support pre-ESM node without eval.

[Raynos]

Wait wtf. I thought import function was just a global function you can check for with type of === undefined

[ljharb]

Absolutely not, it's syntax, not a function.

[Raynos]

Is there a bin feature in npm where you can specify an ESM module bin and a CJS bin ?

Maybe this should be a dual package issue on the npm CLI

[ljharb]

No, there's not - but that's a good feature request for the node ESM modules implementation (although it wouldn't help us to support ESM in the versions already released).

We could do some tricks with a different published package to make it work, perhaps.

[donmccurdy]

unless the project's package.json has "type": "module", which I'd discourage...

By the way, was there a particular reason you'd discourage "type": "module"? Running into an issue (not about Tape) in another project and considering that option...