# Proposal: Range Expression

# Artem Kobzar (10 months ago)

### 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.

``````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.

``````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:

``````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
}
}
``````
# Naveen Chawla (10 months ago)

I like it. I might prefer it in square brackets e.g. [ x ... y ] ?

# Cyril Auburtin (10 months ago)

I'd prefer to have it in slice-notation tc39/proposal-slice-notation#1

``````[...0:8] // [0,1,2,3,4,5,6,7]
``````
# Cyril Auburtin (9 months ago)

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