arraygen

Turn any array into a generator. (More accurately, grab the built-in iterator).

Getting started

For now you'll need a recent version of Node (v6+ works great). This will not work in old browsers or old Node versions unless it gets compiled to ES5. Currently there are no precompiled bundles.

npm install --save arraygen

Why?

I wrote this as an exercise to get more familiar with ES6 generators and iterators. The API is just a more natural (to me) wrapper around Array.prototype.slice(), but instead of returning the raw array, we return the array[Symbol.iterator]().

If this seems silly, it probably is. Let me know if you come up with any compelling use-cases.

Here's what the arr[Symbol.iterator]() allows you to do that you can't do with the ES5 Array API:

let g = arraygen(['c', 'd', 'e'])();
g.next(); // { value: 'c', done: false }
g.next(); // { value: 'd', done: false }
g.next(); // { value: 'e', done: false }
g.next(); // { value: undefined, done: true }

This is the same object type that gets returned from generator functions.

What does that buy us? It means that we can pull values one at a time, perhaps in response to asynchronous events, such as user clicks, network communications, etc...

Currently, I do most of those kinds of things using RxJS, but now that we have native support for something like it (albeit missing most of the cool utility API), maybe there are good use cases to skip the RxJS dependency.

How do you use it?

Destructuring assignment lets you easily grab elements from an array, for example:

let [a, b] = ['a', 'b', 'c'];
console.log(a); // 'a'
console.log(b); // 'b'

You can even use the rest operator to grab all the remaining elements of an array:

let [a, ...rest] = ['a', 'b', 'c'];
console.log(rest); // ['b', 'c']

OK, neat, but wouldn't it be cool if you could specify a range of elements?

For example:

let gen = arraygen(['a', 'b', 'c', 'd', 'e']);
let [...arr] = gen(1, 3); // ['b', 'c', 'd']

This is equivalent to:

let arr = ['a', 'b', 'c', 'd', 'e'].slice(1, 4); // ['b', 'c', 'd']

You could also grab the first n elements:

let gen = arraygen(['a', 'b', 'c', 'd', 'e']);
let [...arr] = gen(3); // ['a', 'b', 'c'];

Equivalent to:

let arr = ['a', 'b', 'c', 'd', 'e'].slice(0, 3); // ['a', 'b', 'c'];

Or the last n elements:

let gen = arraygen(['a', 'b', 'c', 'd', 'e']);
let [...arr] = gen(-3); // ['c', 'd', 'e'];

Equivalent to:

let arr = ['a', 'b', 'c', 'd', 'e'].slice(-3); // ['c', 'd', 'e'];

Grab all elements from start to the last element:

let gen = arraygen(['a', 'b', 'c', 'd', 'e']);
let [...arr] = gen(2, 'tail'); // ['c', 'd', 'e']

Equivalent to:

let arr = ['a', 'b', 'c', 'd', 'e'].slice(2); // ['c', 'd', 'e']

Of course, you can still mix and match any of this with other destructuring assignments:

const gen = arraygen(['a', 'b', 'c', 'd', 'e'])(-3);
const [c, ...rest] = gen(-3);
console.log(`${ c }, ${ JSON.stringify(rest) }`); // c, ["d","e"]

Equvilant to:

let [c, ...rest] = ['a', 'b', 'c', 'd', 'e'].slice(-3);
console.log(`${ c }, ${ JSON.stringify(rest) }`); // c, ["d","e"]

Why not just use Array.prototype.slice()?

As you can see, the synchronous operations are probably better handled with native array.slice(). It's almost certainly less typing, anyway. This slice API is just a convenient way to pre-slice an array while you're grabbing the iterator. I would just use the built-in Array.prototype.slice() to do that, but the API has always bothered me a little.

The alternative slice really belongs in its own module. I've already started to extract it. =)

It might get more compelling if you experiment with using asynchronous events to trigger the .next() method calls... Enjoy. Let me know if you do anything interesting or fun with this.

Written for Learn JavaScript with Eric Elliott

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