Iterables

Iterable objects is a generalization of arrays. That's a concept that allows to make any object useable in a for..of loop.

Of course, Arrays are iterable. But there are many other built-in objects, that are iterable as well. For instance, Strings are iterable also. As we'll see, many built-in operators and methods rely on them.

If an object represents a collection (list, set) of something, then for..of is a great syntax to loop over it, so let's see how to make it work.

Symbol.iterator

We can easily grasp the concept of iterables by making one of our own.

For instance, we have an object, that is not an array, but looks suitable for for..of.

Like a range object that represents an interval of numbers:

let range = {
  from: 1,
  to: 5
};

// We want the for..of to work:
// for(let num of range) ... num=1,2,3,4,5

To make the range iterable (and thus let for..of work) we need to add a method to the object named Symbol.iterator (a special built-in symbol just for that).

1. When for..of starts, it calls that method once (or errors if not found). The method must return an iterator -- an object with the method next.
2. Onward, for..of works only with that returned object.
3. When for..of wants the next value, it calls next() on that object.
4. The result of next() must have the form {done: Boolean, value: any}, where done=true means that the iteration is finished, otherwise value must be the new value.

Here's the full implementation for range:

let range = {
  from: 1,
  to: 5
};

// 1. call to for..of initially calls this
range[Symbol.iterator] = function() {

  // ...it returns the iterator object:
  // 2. Onward, for..of works only with this iterator, asking it for next values
  return {
    current: this.from,
    last: this.to,      

    // 3. next() is called on each iteration by the for..of loop
    next() {
      // 4. it should return the value as an object {done:.., value :...}
      if (this.current <= this.last) {
        return { done: false, value: this.current++ };
      } else {
        return { done: true };
      }
    }
  };
};

// now it works!
for (let num of range) {
  alert(num); // 1, then 2, 3, 4, 5
}

Please note the core feature of iterables: an important separation of concerns:

• The range itself does not have the next() method.
• Instead, another object, a so-called "iterator" is created by the call to range[Symbol.iterator](), and it handles the whole iteration.

So, the iterator object is separate from the object it iterates over.

Technically, we may merge them and use range itself as the iterator to make the code simpler.

Like this:

let range = {
  from: 1,
  to: 5,

  [Symbol.iterator]() {
    this.current = this.from;
    return this;
  },

  next() {
    if (this.current <= this.to) {
      return { done: false, value: this.current++ };
    } else {
      return { done: true };
    }
  }
};

for (let num of range) {
  alert(num); // 1, then 2, 3, 4, 5
}

Now range[Symbol.iterator]() returns the range object itself: it has the necessary next() method and remembers the current iteration progress in this.current. Shorter? Yes. And sometimes that's fine too.

The downside is that now it's impossible to have two for..of loops running over the object simultaneously: they'll share the iteration state, because there's only one iterator -- the object itself. But two parallel for-ofs is a rare thing, doable with some async scenarios.

 Infinite iterators are also possible. For instance, the range becomes infinite for range.to = Infinity. Or we can make an iterable object that generates an infinite sequence of pseudorandom numbers. Also can be useful.

There are no limitations on next, it can return more and more values, that's normal.

Of course, the for..of loop over such an iterable would be endless. But we can always stop it using break.

String is iterable

Arrays and strings are most widely used built-in iterables.

For a string, for..of loops over its characters:

for (let char of "test") {
  // triggers 4 times: once for each character
  alert( char ); // t, then e, then s, then t
}

And it works correctly with surrogate pairs!

Calling an iterator explicitly

Normally, internals of iterables are hidden from the external code. There's a for..of loop, that works, that's all it needs to know.

But to understand things a little bit deeper let's see how to create an iterator explicitly.

We'll iterate over a string the same way as for..of, but with direct calls. This code gets a string iterator and calls it "manually":

let str = "Hello";

// does the same as
// for (let char of str) alert(char);

let iterator = str[Symbol.iterator]();

while (true) {
  let result = iterator.next();
  if (result.done) break;
  alert(result.value); // outputs characters one by one
}

That is rarely needed, but gives us more control over the process than for..of. For instance, we can split the iteration process: iterate a bit, then stop, do something else, and then resume later.

Iterables and array-likes

There are two official terms that look similar, but are very different. Please make sure you understand them well to avoid the confusion.

• Iterables are objects that implement the Symbol.iterator method, as described above.
• Array-likes are objects that have indexes and length, so they look like arrays.

Naturally, these properties can combine. For instance, strings are both iterable (for..of works on them) and array-like (they have numeric indexes and length).

But an iterable may be not array-like. And vice versa an array-like may be not iterable.

For example, the range in the example above is iterable, but not array-like, because it does not have indexed properties and length.

And here's the object that is array-like, but not iterable:

let arrayLike = { // has indexes and length => array-like
  0: "Hello",
  1: "World",
  length: 2
};

*!*
// Error (no Symbol.iterator)
for (let item of arrayLike) {}
*/!*

What do they have in common? Both iterables and array-likes are usually not arrays, they don't have push, pop etc. That's rather inconvenient if we have such an object and want to work with it as with an array.

Array.from

There's a universal method Array.from that brings them together. It takes an iterable or array-like value and makes a "real" Array from it. Then we can call array methods on it.

For instance:

let arrayLike = {
  0: "Hello",
  1: "World",
  length: 2
};

*!*
let arr = Array.from(arrayLike); // (*)
*/!*
alert(arr.pop()); // World (method works)

Array.from at the line (*) takes the object, examines it for being an iterable or array-like, then makes a new array and copies there all items.

The same happens for an iterable:

// assuming that range is taken from the example above
let arr = Array.from(range);
alert(arr); // 1,2,3,4,5 (array toString conversion works)

The full syntax for Array.from allows to provide an optional "mapping" function:

Array.from(obj[, mapFn, thisArg])

The second argument mapFn should be the function to apply to each element before adding to the array, and thisArgallows to set this for it.

For instance:

// assuming that range is taken from the example above

// square each number
let arr = Array.from(range, num => num * num);

alert(arr); // 1,4,9,16,25

Summary

Objects that can be used in for..of are called iterable.

• Technically, iterables must implement the method named Symbol.iterator.
  • The result of obj[Symbol.iterator] is called an iterator. It handles the further iteration process.
  • An iterator must have the method named next() that returns an object {done: Boolean, value: any}, here done:true denotes the iteration end, otherwise the value is the next value.
• The Symbol.iterator method is called automatically by for..of, but we also can do it directly.
• Built-in iterables like strings or arrays, also implement Symbol.iterator.
• String iterator knows about surrogate pairs.

Objects that have indexed properties and length are called array-like. Such objects may also have other properties and methods, but lack the built-in methods of arrays.

If we look inside the specification -- we'll see that most built-in methods assume that they work with iterables or array-likes instead of "real" arrays, because that's more abstract.

Array.from(obj[, mapFn, thisArg]) makes a real Array of an iterable or array-like obj, and we can then use array methods on it. The optional arguments mapFn and thisArg allow us to apply a function to each item.