diff --git a/1-js/05-data-types/06-iterable/article.md b/1-js/05-data-types/06-iterable/article.md index 6611d4dc..7f72f5ca 100644 --- a/1-js/05-data-types/06-iterable/article.md +++ b/1-js/05-data-types/06-iterable/article.md @@ -3,22 +3,11 @@ *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 we can make any objects iterable. That helps to represent *collections* of values. - -Iterables are widely used by the core JavaScript. For instance, Strings are iterable also. As we'll see many built-in operators and methods rely on them. - -Here's an example of iteration over a string: - -```js run -for (let char of "test") { - alert( char ); // t, then e, then s, then t -} -``` - -As we can see, `for..of` treats string as a collection of characters, and iterates over them. +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. @@ -39,10 +28,10 @@ let range = { 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). -- 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`. -- When `for..of` wants the next value, it calls `next()` on that object. -- 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. +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`: @@ -55,7 +44,8 @@ let range = { // 1. call to for..of initially calls this range[Symbol.iterator] = function() { - // 2. ...it returns the iterator: + // ...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, @@ -78,10 +68,10 @@ for (let num of range) { } ``` -There is an important separation of concerns in this code: +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 iteration. +- 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. @@ -113,10 +103,12 @@ for (let num of range) { } ``` -Now `range[Symbol.iterator]()` returns the `range` object itself: it has the necessary `next()` method and remembers the current iteration progress in `this.current`. 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. +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. ```smart header="Infinite iterators" -Infinite iterators are also doable. 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. +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. @@ -130,9 +122,14 @@ Arrays and strings are most widely used built-in iterables. For a string, `for..of` loops over its characters: +```js run +for (let char of "test") { + // triggers 4 times: once for each character + alert( char ); // t, then e, then s, then t +} +``` - -And it works right with surrogate pairs! +And it works correctly with surrogate pairs! ```js run let str = '𝒳😂';