fix: early return when instance is not iterable

[JLHwung]

Q	A
Fixed Issues?	Fixes #9210
Patch: Bug Fix?	Yes
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

The slicedToArray is supposed to throw nonIterableRest error when applied to an non-iterable instance. However, the iterableToArrayLimit will always throw error in arr[Symbol.iterator] when arr is not iterable.

Here we early return in iterableToArrayLimit so that we can offer better error message in runtime. Note that by doing so we also align iterableToArrayLimit with iterableToArray.

[JLHwung]

The iterableToArray has guarded this check with an arguments check

export default function _iterableToArray(iter) {
    if (
      Symbol.iterator in Object(iter) ||
      Object.prototype.toString.call(iter) === "[object Arguments]"
    ) return Array.from(iter);
  }

But

(function () { return Symbol.iterator in Object(arguments) })()

returns true in Chrome 76. I am not sure if this behavior is inconsistent across different browsers.

[JLHwung]

Oh, there is a browser support matrix on arguments[@@iterator]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments/@@iterator

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/11483/

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/11471/

Actually yes, we need to check for arguments. This is because arguments can't be polyfilled (becaue it is re-created every time), so Symbol.iterator in Object(arguments) will always be false in old browsers, even with a polyfill.