Proposal: Range Expression

Cyril Auburtin cyril.auburtin at gmail.com
Mon Aug 19 08:29:27 UTC 2019


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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>>
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