Python-like range function in Javascript.

range.js

/**
 * Python-like range function
 * 
 * @param start {Number} Start value, inclusive, if end isn't provided, start is end
 * @param end {Number|null} End value, exclusive, defaults to null
 * @param step {Number} Step value, default to 1
 * @returns {Array} [start, start+step, start+step*2, ..., end - 1]
 */
const range = (start, end=null, step=1) => {
	if (end == null) {
		// start is actually end because 1 arg provided
		return [...Array(start).keys()];
	}
	
	if (end <= start && step > 0) {
		return [];
	}
	
	if (step < 0) {
		return range(end + 1, start + 1, -step).reverse();
	}
	
	return [...Array(end).keys()].filter(i => i > start - 1).filter(i => i % step == 0);
};

module.exports = range;

/* Usage:
for (let x of range(6)) {
	console.log(x); // 0, 1, 2, 3, 4, 5
}

for (let x of range(5, 7)) {
	console.log(x); // 5, 6
}

for (let x of range(0, 10, 2)) {
	console.log(x); // 0, 2, 4, 6, 8
}

for (let x of range(6, 1, -1)) {
	console.log(x); // 6, 5, 4, 3, 2
}

for (let x of range(6, 1, -2)) {
	console.log(x); // 6, 4, 2
}
*/