Why for..of does not log the foo value? [duplicate]

Question

I am looking at a sample from mozilla documents about JS and here is this snippet that shows that For..of prints 3, 5, 7, but why not "hello"?

let arr = [3, 5, 7]; arr.foo = "hello"; for (let i in arr) { console.log(i); // logs "0", "1", "2", "foo" } for (let i of arr) { console.log(i); // logs "3", "5", "7" ????? }

You've copied the examples from MDN, however you didn't read what it says, While for...in iterates over property names, for...of iterates over property values:. hello is the value of the key foo. — Tushar
– Tushar, Commented Feb 2, 2016 at 8:47
And there is a a property called "foo" with a value "hello". Why doesn't it show it? — v.g.
– v.g., Commented Feb 2, 2016 at 8:48
@Tushar Did you try the code? "Hello" is indeed not printed. — Lewis
– Lewis, Commented Feb 2, 2016 at 8:50
Yes, and my question is Why? Isn't foo the fourth element of arr? — v.g.
– v.g., Commented Feb 2, 2016 at 8:51
@v.g.: No. arr[3] is undefined. arr.foo is just an arbitrary property of the array, like arr.length is, it's not an array element. Only integer-indexed properties are. — Bergi
– Bergi, Commented Feb 2, 2016 at 8:52

Community · Accepted Answer · 2017-05-23 12:31:08Z

Arrays are iterables over their elements. That's how it's defined. That is how Array[Symbol.iterator] is implemented.

See this.

Source.

EDIT:
foo is property. 3, for example, is an element but "hello" isn't. Elements are the values that occur as values of integer-valued properties.

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