Use of enums is transpiled in a different way than with tsc

For some reason TypeScript enum values are transpiled differently with ts-node than with tsc. This can lead to runtime errors if the used enum doesn’t exist at runtime.

Here’s an example: the current typings for big.js define an enum that doesn’t actually exist in the JS library:

// node_modules/@types/big.js/index.d.ts
export const enum Comparison {
    GT = 1,
    EQ = 0,
    LT = -1,
}

This enum doesn’t exist at runtime:

console.info(require('big.js').Comparison)
// undefined

Now, let’s say we have this TS code:

import {Comparison} from 'big.js'

Comparison.LT.toString()

tsc transpiles it to this (using a sane tsconfig.json with ES2017 target):

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
-1 /* LT */.toString();

Note that there’s no require calls anywhere and the enum value -1 is inlined with a comment included. Running the generated JS file with node doesn’t throw any exceptions.

Now, running the same TS file with ts-node gives me this:

/home/joonas/enum-bug/main.ts:3
Comparison.LT.toString()
           ^
TypeError: Cannot read property 'LT' of undefined
    at Object.<anonymous> (/home/joonas/enum-bug/main.ts:3:12)
    at Module._compile (module.js:643:30)
    at Module.m._compile (/home/joonas/enum-bug/node_modules/ts-node/src/index.ts:423:6)
    at Module._extensions..js (module.js:654:10)
    at Object.require.extensions.(anonymous function) [as .ts] (/home/joonas/enum-bug/node_modules/ts-node/src/index.ts:426:4)
    at Module.load (module.js:556:32)
    at tryModuleLoad (module.js:499:12)
    at Function.Module._load (module.js:491:3)
    at Function.Module.runMain (module.js:684:10)
    at Object.<anonymous> (/home/joonas/enum-bug/node_modules/ts-node/src/_bin.ts:177:12)

After adding some logging to ts-node inside my node_modules, I can see that this is the transpiled output:

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
const big_js_1 = require("big.js");
big_js_1.Comparison.LT.toString();

Note the require call and how the enum value is not inlined but accessed at runtime.

Any ideas? This might of course be a misconfiguration in my part, a TS compiler bug, or a problem with the compiler API.

$ node --version
v8.9.4

Library versions:

  "dependencies": {
    "@types/big.js": "4.0.2",
    "big.js": "5.0.3",
    "ts-node": "4.1.0",
    "typescript": "2.6.2"
  }

[svkurowski]

Hello, please re-open this issue and remove the invalid label as ts-node is behaving differently than tsc and node for completely valid TypeScript code. Here is a minimal example to reproduce:

MyEnum.ts:

const enum MyEnum {
    A = "A",
    B = "B",
}
export default MyEnum;

index.ts:

import MyEnum from "./MyEnum";
console.log(MyEnum.A)

Expected behavior as evidenced by tsc/node:

$ node_modules/.bin/tsc index.ts MyEnum.ts
$ node index.js
A

Actual behavior of ts-node:

$ node_modules/.bin/ts-node index.ts

index.ts:2
console.log(MyEnum.A)
                   ^
TypeError: Cannot read property 'A' of undefined
    at Object.<anonymous> (index.ts:3:20)
    at Module._compile (module.js:660:30)
    at Module.m._compile (node_modules/ts-node/src/index.ts:400:23)
    at Module._extensions..js (module.js:671:10)
    at Object.require.extensions.(anonymous function) [as .ts] (node_modules/ts-node/src/index.ts:403:12)
    at Module.load (module.js:573:32)
    at tryModuleLoad (module.js:513:12)
    at Function.Module._load (module.js:505:3)
    at Function.Module.runMain (module.js:701:10)
    at Object.<anonymous> (node_modules/ts-node/src/bin.ts:141:12)

Yes, it works correctly with the TS_NODE_TYPE_CHECK environment variable set to Y but this issue should still be fixed.

[electricessence]

Oddly… I ran my tests against pure typescript as well (no JS) and const enums still did not propagate correctly. I had to set “preserveConstEnums” to true.