TypeScript and Iterator: Type 'IterableIterator<T>' is not an array type

Question

When I use the yield* expression on TypeScript, it always gets an error.

Type 'IterableIterator' is not an array type.

How can I set the types correctly without using any to avoid the errors?

function* g1(): IterableIterator<number> {
  yield 2;
  yield 3;
  yield 4;
}

function* g2(): IterableIterator<number> {
  yield 1;
  // ERROR: Type 'IterableIterator<number>' is not an array type.
  yield* g1();
  yield 5;
}

const iterator = g2();

Answer 1

If possible update target to es2015 (or above) in the tsconfig to resolve the error without enabling down level iteration`:

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

If you target es5, enable downLevelIteration explicitly in the tsconfig:

{
  "compilerOptions": {
    "target": "es5",
    "downlevelIteration": true
  }
}