On Sun, Mar 25, 2018 at 6:20 PM, 月の影 <19511...@qq.com> wrote:
> Sometimes I got a infinity iterable sequences, I may want a partial spreed
> syntax. `...iterableObject{from, to}`
>
> For example:
>
> ```js
> function *fibonacci() {
> let a = 0, b = 1
> while(1) {
> [a, b] = [b, a + b]
> yield a
> }
> }
>
> console.log([...fibonacci(){3, 5}]) // [3, 5, 8]
> ```
>
> For a finite list, it is similar to [...list].slice(from, to)
>
> ```js
> const arr1 = [1, 2, 3, 4],
> arr2 = [5, 6, 7, 8]
>
> console.log([...arr1{2}, ...arr2{1}]) // 3, 4, 6, 7, 8
> ```
This seems to be well-served by a simple library function right now:
```
function take(n, iter) {
if(n <= 0) return;
for(let item of iter) {
yield item;
if(n-- <= 0) return;
}
}
function drop(n, iter) {
iter = iter[Symbol.iterator];
for(let i = 0; i < n; i++) {
iter.next();
}
yield* iter;
}
function subiter(start, end, iter) {
return take(end-start+1, drop(start, iter));
}
console.log([...subiter(3, 5, fibonacci())]);
```
~TJ
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