"__assign is not defined" - NativeScript Object Spread Woes

Question

I was making good progress with a native-script project until this happened:

JS: EXCEPTION: Uncaught (in promise): ReferenceError: __assign is not defined

This is bubbling up from this line of code:

return [...state, { ...action.payload, success: false }];

Here's my tsconfig.json

{
    "compilerOptions": {
        "module": "commonjs",
        "target": "es5",
        "experimentalDecorators": true,
        "emitDecoratorMetadata": true,
        "noEmitHelpers": true,
        "noEmitOnError": true
    },
    "exclude": [
        "node_modules",
        "platforms",
        "**/*.aot.ts"
    ]
}

Typescript doesn't seem to be including it's helper __assign function in the compiled source - which is their way of implementing the object spread syntax. Would any of you fine people happen to know why?

Answer 1

I'm happy to report I found the solution to this. This GitHub repo explains things quite nicely, but here's a quick rundown:

The flag noEmitHelpers in tsconfig.json tells Typescript to omit these 'helpers' (such as __assign) in every file that needs them.

{
  "compilerOptions": {
    // changing this to false does the job, but duplicates helpers across every file
    "noEmitHelpers": false
  }
}

The latest Typescript offers a better way to manage this, using the flag importHelpers (see compiler options):

{
  "compilerOptions": {
    "noEmitHelpers": true,
    "importHelpers": true // better
  }
}

This'll get object spread working, and avoid code duplication across files.

You might also need to npm install tslib --save to stop IDE errors.