What is the technical definition of a Javascript iterable and how do you test for it?

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What is the real definition of a Javascript iterable using the term as the ES6 specification does?

§25.1.1.1 defines “The Iterable Interface”.

They’re objects with a Symbol.iterator-keyed method that returns a valid Iterator (which in turn is an object expected to behave as it should according to §25.1.1.2).

How do you test for it in ES6 Javascript?

We cannot test what the @@iterator method returns without calling it, and we cannot test whether the result conforms to the Iterator interface without trying to run it. The best bet would be to do

function looksIterable(o) {
    return typeof o[Symbol.iterator] == "function";
}

however I wouldn’t usually test for this but simply let it fail with an exception when it’s not iterable.

How should you iterate a generic iterable?

Don’t use forEach. (In fact, never use forEach anywhere in ES6).

The proper way to iterate is a for (… of …) loop. It does all the checking for iterability (using the abstract GetIterator operation and running (and even closing) the iterator, and throws appropriate TypeErrors when used on non-iterable values.

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