Iterables

loops over an iterable object, invoking the Symbol.iterator property on the object. Here is a simple for..of loop on an array:

Both for..of and for..in statements iterate over lists; the values iterated on are different though, for..in returns a list of keys on the object being iterated, whereas for..of returns a list of values of the numeric properties of the object being iterated.

Another distinction is that for..in operates on any object; it serves as a way to inspect properties on this object. for..of on the other hand, is mainly interested in values of iterable objects. Built-in objects like and Set implement Symbol.iterator property allowing access to stored values.

Code generation

Targeting ES5 and ES3

When targeting an ES5 or ES3-compliant engine, iterators are only allowed on values of Array type. It is an error to use for..of loops on non-Array values, even if these non-Array values implement the Symbol.iterator property.

will be generated as:

Targeting ECMAScript 2015 and higher

When targeting an ECMAScipt 2015-compliant engine, the compiler will generate for..of loops to target the built-in iterator implementation in the engine.