ES6/7 equivalent to underscore’s range function

The idea is to create an array of length end - start + 1, and then fill it with the relevant numbers using Array#from.

The Array.from() method creates a new Array instance from an array-like or iterable object.

In this case Array#from needs an object with the length property. Using Array(n) creates such an object (array). You can also use { length: n } directly. In this case n = Math.abs(end - start) + 1.

Array#from accepts a mapFn callback, a function that can transform the iterated value. The function receives 2 params - the values (which we can ignore in this case), and the index (0 based). Adding start to the current index will create the numbers in the range.

const range = (start, end) => Array.from(
  Array(Math.abs(end - start) + 1), 
  (_, i) => start + i
);

console.log(range(5, 10));
console.log(range(-10, -5));
console.log(range(-5, 10));

This version will handle reverse range as well (large to small) as well:

const range = (start, end) => {
  const inc = (end - start) / Math.abs(end - start);
  
  return Array.from(
    Array(Math.abs(end - start) + 1), 
    (_, i) => start + i * inc
  );
};

console.log(range(5, 10));
console.log(range(-5, -10));
console.log(range(10, -5));

Note that the following implementation does not allow for lazy-generated lists:

Array.from(Array(n), (_, i) => x + i)

Imagine that you need a list of 1M numbers:

range(1, 1000000);

Are you going to consume all of them? Maybe not, yet all the numbers have been generated already and they probably left a non-trivial footprint on your memory.

It would be nice if we could get them one by one on demand.

It turns out we can do exactly that with a generator:

function* range(start, end, step = 1) {
  for (let value = start; value < end; value += step) {
    yield value;
  }
}

for (let x of range(1, Infinity)) {
  if (x > 10) {
    break;
  }
  console.log(x);
}

Did you notice the range(1, Infinity) bit?

In a classic implementation where all the numbers are generated in advance, this code wouldn't even run as you would get stuck in an infinite number-generating loop.