Using JavaScript generator functions for ranges

Generator functions

JavaScript ES6 generators allow you to define functions that can be exited and later re-entered, while retaining their context (variable bindings). They are defined using function* (function keyword followed by an asterisk) and use yield expressions to return their result. For example:

function* generateRange(end, start = 0, step = 1) {
  let x = start - step;
  while(x < end - step) yield x += step;
}

const gen5 = generateRange(5);
let x = gen5.next();

while (!x.done) {
  console.log(x.value);
  x = gen5.next();
} // Logs: 0, 1, 2, 3, 4

In the above example, we define a generator function, generateRange, which will return each value between start and end, incrementing by step each time. We use the generator object to call Generator.prototype.next() until it returns {value: undefined, done: true} to iterate over the values the generator produces.

Symbol.iterator

Symbol.iterator specifies the default iterator for an object. Oftentimes, Symbol.iterator is implemented using a generator function. For example:

const iterableXx = {
  [Symbol.iterator]: function* () {
    yield 1;
    yield 2;
  }
};

console.log([...iterableX]); // [1, 2]

As you can see in this example, the object is made iterable by assigning a generator function to its Symbol.iterator property. This can come especially handy, if you want to iterate over some arbitrary data or create an object that is iterable and uses a generator function under the hood.

Putting it all together

Knowing how both concepts work, we can combine them to create a range generator, similar to Python or Ruby's ranges:

const range = (end, start = 0, step = 1) => {
  function* generateRange() {
    let x = start - step;
    while(x < end - step) yield x += step;
  }
  return {
    [Symbol.iterator]: generateRange
  };
}

console.log([...range(7)]); // [0, 1, 2, 3, 4, 5, 6]
for (let i of range(8, 2, 2)) console.log(i); // Logs: 2, 4, 6