Constructor error (TS2351) in TypeScript when using ESM since v5

[abouvier]

package.json

{
  "type": "module",
  "dependencies": {
    "fraction.js": "^5.1.0"
  },
  "devDependencies": {
    "typescript": "^5.6.3"
  }
}

tsconfig.json

{
  "compilerOptions": {
    "module": "node16"
  }
}

test.ts

import Fraction from "fraction.js";

let x = new Fraction(1.88);

Compilation error:

$ npx tsc
test.ts:3:13 - error TS2351: This expression is not constructable.
  Type 'typeof import(".../node_modules/fraction.js/fraction")' has no construct signatures.

3 let x = new Fraction(1.88);
              ~~~~~~~~


Found 1 error in test.ts:3

It worked fine in v4, and it works again if "type": "module" is added back to node_modules/fraction.js/package.json.

[infusion]

What is your current nodejs version? For me it worked providing just the TS definition file. From v4 to v5 the environment setup was changed to work on all major platforms. The "type": "module" is something that makes a module a 100% ESM module, which breaks all older installations. Does it work with a named export?

import {Fraction} from "fraction.js";

[abouvier]

What is your current nodejs version?

I tested with nodejs version 22.9.0 and 23.1.0.

Does it work with a named export?

Nope:

$ npx tsc
test.ts:1:9 - error TS2614: Module '"fraction.js"' has no exported member 'Fraction'. Did you mean to use 'import Fraction from "fraction.js"' instead?

1 import {Fraction} from "fraction.js";
          ~~~~~~~~


Found 1 error in test.ts:1

By removing default on the line export default class Fraction { in node_modules/fraction.js/fraction.d.ts then it works with import {Fraction} from "fraction.js";

[infusion]

That's an interesting insight, thanks! I tested it again and again with nodejs from v16 to v23 and never had a problem. However, since your hint is very valuable, I updated the .d.ts file to support named and default export, same as the ESM export. Hope that also fixes your issue!

[abouvier]

That's an interesting insight, thanks! I tested it again and again with nodejs from v16 to v23 and never had a problem. However, since your hint is very valuable, I updated the .d.ts file to support named and default export, same as the ESM export. Hope that also fixes your issue!

Yes now import {Fraction} from "fraction.js"; is working, but the default still not.

It works though if fraction.d.ts is renamed to fraction.d.mts (and the name updated in package.json).
As explained here, each file extension should reflect the type of module used in the file, because without "type": "module" in package.json then fraction.d.ts is considered a CommonJS file.

[abouvier]

Are the types wrong?

Explanation of the problem: https://github.com/arethetypeswrong/arethetypeswrong.github.io/blob/main/docs/problems/FalseCJS.md

[infusion]

Thanks for the test tool. I couldn't grasp the problem of the issue yet, since I'm not a TS developer, but still want to maintain the highest compatibility. Is it generally possible to give TS the correct exports for CJS and MJS? "type": "module" is not possible, since it would break the CommonJS import. The solution in the JS world is to maintain the exports field in the package.json. After a lot of trouble this now works perfectly fine, the question is how the TS compiler can get the right definition for each regime.

[abouvier]

You probably need to maintain one type file for each engine, like in this library for example:

MikeMcl/bignumber.js@a543ab5
MikeMcl/bignumber.js#371