An error in TypeScript has occurred, stating that the type 'IterableIterator<any>' is neither an array type nor a string type

Hey there! I'm currently working on an Angular project and encountering an error in my VS Code. I'm a bit stuck on how to fix it.

    Type 'IterableIterator<any>' is not an array type or a string type.
 Use compiler option '--downlevelIteration' to allow iterating of iterators.ts(2569)

Below is the snippet of the code causing the issue:

  aggregator(data: HistogramDistribution[]) {
   data.forEach(
      item => {
       item.dateRange =  moment.utc(item.dateRange).local().format("YYYY-MM-DD").toString();
      }
    );
 
    return [...data
      .reduce((acc, o) => {
        
        const key = o.dateRange,
              group = acc.get(key)
        group ?
        (group.total += o.total, 
        group.delivered += o.delivered, 
        group.undeliverable += o.undeliverable,
        group.expired += o.expired,
        group.enroute += o.enroute) :
        acc.set(key, {...o, dateRange: key})
        return acc
      }, new Map)
      .values()
   ];
  }

I've attempted adding "downlevelIteration": true to my tsconfig.json but the error persists.

{
  "compileOnSave": false,
  "compilerOptions": {
    "outDir": "./dist/out-tsc",
    "sourceMap": true,
    "declaration": false,
    "moduleResolution": "node",
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true,
    "target": "es5",
    "typeRoots": ["node_modules/@types"],
    "lib": ["es2017", "dom"],
    "downlevelIteration": true
  }
}

Would really appreciate any assistance you can provide.

Thank you!

Answer №1

To resolve the issue, simply adjust your compile target to es6 or higher within the tsconfig.json configuration file.

{
  "compilerOptions": {
    "target": "es2015"
  }
}

If you prefer to stick with an older version, you can enable downLevelIteration by following these steps

{
  "compilerOptions": {
    "target": "es2015",
    "downlevelIteration": true
   }
}

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