swc transpiler does not set `strictMode` to match tsconfig

[Janpot]

Search Terms

swc

Expected Behavior

Running

function defaultHello () {
  return ''
}

class X {
  constructor(x) {
    this.hello = () => (x.hello || defaultHello)()
  }
}

console.log(new X({
  hello () {
    return this.y || 'this works'
  }
}).hello())

using

  "ts-node": {
    "transpileOnly": true,
    "transpiler": "ts-node/transpilers/swc-experimental"
  },

will print

this works

(this is what is printed without using swc)

Actual Behavior

an error is printed:

/swctest/index.ts:13
    return this.y || 'this works'
                ^
TypeError: Cannot read property 'y' of undefined
    at hello (/swctest/index.ts:13:17)
    at X.hello (/swctest/index.ts:7:50)
    at Object.<anonymous> (/swctest/index.ts:15:4)
    at Module._compile (internal/modules/cjs/loader.js:1063:30)
    at Module.m._compile (/swctest/node_modules/ts-node/src/index.ts:1371:23)
    at Module._extensions..js (internal/modules/cjs/loader.js:1092:10)
    at Object.require.extensions.<computed> [as .ts] (/swctest/node_modules/ts-node/src/index.ts:1374:12)
    at Module.load (internal/modules/cjs/loader.js:928:32)
    at Function.Module._load (internal/modules/cjs/loader.js:769:14)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:72:12)

Specifications

ts-node v10.4.0
node v14.16.0
compiler v4.4.4
@swc/core 1.2.103 

• tsconfig.json, if you're using one:

{
  "compilerOptions": {
    "target": "es5",                 
    "module": "commonjs",
    "esModuleInterop": true, 
    "forceConsistentCasingInFileNames": true, 
    "strict": false,  
    "skipLibCheck": true },

  "ts-node": {
    "transpileOnly": true,
    "transpiler": "ts-node/transpilers/swc-experimental"
  },
}

• Operating system and version: MacOS 11.6

[cspotcode]

Do you think this is a strict mode issue? E.g swc is enabling strict mode when you expect it to be disabled, or vice versa? I'm referring specifically to https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode, not to TypeScripts's "strict" option which is a different thing.

What does this do?

console.log(new X({
  hello () {
    console.dir(this); // figure out the `this` value.  Is it equal to `global`?
  }
}).hello())

[Janpot]

Ah ofcourse, yes indeed, it's the global object, swc seems to add the use strict pragma. Manually adding it makes the behavior the same when I disable swc.
Aside, I've been trying to use the --emit flag to inspect the transpiler output, but it doesn't seem to have any effect. Am I misunderstanding the purpose of this flag?

[cspotcode]

Ok, looks we do not pass the strictMode flag to swc and it defaults to true.
https://swc.rs/docs/config-js-module#strictmode

ts-node/src/transpilers/swc.ts

Lines 85 to 90 in bc03a3e

	module: moduleType
	? ({
	noInterop: !esModuleInterop,
	type: moduleType,
	} as swcTypes.ModuleConfig)
	: undefined,

We should be explicitly setting this option to match whatever your typescript config indicates, so that you'll get the same behavior with both compilers.
https://www.typescriptlang.org/tsconfig#alwaysStrict

I've renamed this issue to describe the underlying problem, which helps keep track of implementing a fix.

---

Re the --emit flag, it is meant to write all compiled output into a .ts-node directory. But it's possibly non-functional when combined with transpilers / transpile-only? I'm not sure.

I also have this idea to add a flag that is specifically for inspecting the compiled output of a single file at a time, for debugging situations like this: #1366

[Janpot]

Re the --emit flag, it is meant to write all compiled output into a .ts-node directory. But it's possibly non-functional when combined with transpilers / transpile-only? I'm not sure.

It never seems to emit for me, regardless of the options

I also have this idea to add a flag that is specifically for inspecting the compiled output of a single file at a time, for debugging situations like this: #1366

Personally, I think I'd still use just --emit

[cspotcode]

If you think --emit is not working correctly, feel free to submit a bug report. Please check for pre-existing reports first.

[cspotcode]

@Janpot What is your tsconfig? There are various options which affect "use strict" and I want to make sure I understand your project configuration 100%. Thank you in advance.

---

I did a bit of research here. tsc, by default, will add "use strict" whenever you use any sort of import or export. Additionally, if strict: true is configured, it will always emit "use strict".

Your script above is a special case. I expect that most node scripts, even standalone ones, will have at least one import, for example import {} from 'fs'; TS is bending over backwards to allow scripts with zero imports or exports to run in non-strict mode. I suspect this behavior only exists for backwards compatibility, and you generally always want "use strict" enabled when working with node.

swc is not as nuanced as TS. We can either tell it to always emit use strict, or never emit.

Given this limitation of swc, I'm going to pragmatically assume that all node scripts have an import or export, and set swc's options accordingly.

If a user really really needs to avoid strict mode, they will likely have alwaysStrict: false, noImplicitUseStrict: true. We can detect this and tell swc not to emit "use strict"

[cspotcode]

Nevermind... of course you included your tsconfig at the top, doh!

[Janpot]

This was originally discovered in a repo that was transitioning from js to ts. all the imports were require calls. not all files had a 'use strict' pragma. the original js files were not compiled before and after running them through ts-node, this issue came up.

[cspotcode]

Thanks, good to understand the context. I think the implementation we settled on is the best we can do, given that swc's configuration is a bit more limited than tsc's.