Difference between for...of and for...in

The for...in loop will iterate over all enumerable properties of an object.
The for...of syntax is specific to collections, rather than all objects. It will iterate in this manner over the elements of any collection that has a [Symbol.iterator] property.
The following example shows the difference between a for...of loop and a for...in loop.

Object.prototype.objCustom = function () {};
Array.prototype.arrCustom = function () {};

let iterable = [3, 5, 7];
iterable.foo = "hello";

for (let i in iterable) {
     console.log(i);  // logs 0, 1, 2, "foo", "arrCustom", "objCustom"
}
for (let i of iterable) {
     console.log(i);  // logs 3, 5, 7 
}

@reference_0_mozilla