Proposal: Range Expression

Cyril Auburtin cyril.auburtin at gmail.com
Tue Aug 20 16:28:47 UTC 2019


Here is a PR for adding SliceExpression to proposal-slice-notation (that
could probably be renamed proposal-slice-expression if it's accepted)
https://github.com/tc39/proposal-slice-notation/pull/32

On Mon, Aug 19, 2019 at 10:29 AM Cyril Auburtin <cyril.auburtin at gmail.com>
wrote:

> I'd prefer to have it in slice-notation
> https://github.com/tc39/proposal-slice-notation/issues/1#issuecomment-522227270
>
> ```js
> [...0:8] // [0,1,2,3,4,5,6,7]
> ```
>
> On Mon, Aug 19, 2019 at 9:34 AM Naveen Chawla <naveen.chwl at gmail.com>
> wrote:
>
>> I like it. I might prefer it in square brackets e.g. [ x ... y ] ?
>>
>> On Sun, 18 Aug 2019 at 23:24, Artem Kobzar <a.kobzar.nlt at gmail.com>
>> wrote:
>>
>>> ### Proposal: Range Expression
>>>
>>> The proposal based on one of the worst things in JavaScript Arrays.
>>>
>>> If i as developer want to make sequence of some numbers or characters
>>> then i need to use weird methods like `Array.from`, manual filling or
>>> imperative-style array filling.
>>>
>>> ```js
>>> const group1 = [1, 2, 3, 4, 5, ...]// manual filling
>>> const group2= Array.from({ length: 15 }).map((_, i) => i + 1);
>>> const group3 = [];
>>> for (let i = 0; i < 15; i++) group3[i] = i + 1;
>>> ```
>>>
>>> This ways to describe range sequence is huge and unreadable.
>>>
>>>
>>> So, my proposal is: range expression which help to create lazy iterable
>>> range sequence in declarative way.
>>>
>>> ```js
>>> const group0 = 1...15; // from 1 to 15
>>> const group1 = 15...1; // from 15 to 1
>>> const group2 = 1n...15n // from 'a' to 'z'
>>> const group3 = 'a'...'z' // from 'a' to 'z'
>>> const group4 = 82... // lazy collection of numbers
>>>
>>> for (const odd of 1...85) {
>>>  // ...
>>> }
>>>
>>> // in combination with https://github.com/tc39/proposal-iterator-helpers
>>> const allEven = (1...100).filter(isEven);
>>> ```
>>>
>>> So, this operator is syntax sugar for the next generator which create
>>> lazy iterable sequence:
>>>
>>> ```js
>>> function* range(start, end, second) {
>>>   validate(start, end, second)// type assertion of start, second and end
>>>   const step = difference(start, second); // difference calculation
>>>   let current = start;
>>>   while(isNotEnd(current, end)) { // logic for negative step and
>>> different types
>>>     yield current;
>>>     current = next(current, step); // function which change current
>>> number | bigint | string by difference
>>>   }
>>> }
>>> ```
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