Call join() on a generator in JS

Question

In Python, you can call the string.join() method on any iterable, like so:

",".join(some_iterable)

The argument may be a list, generator, or any other object as long as it is iterable.

Playing around with ES6, I couldn't find a way to do so without having to create an array first, I had to do something like this:

function *myGenerator() { ... }
let output = [...myGenerator()].join(",");

I know that join() is an Array.prototype method. Is it possible for me to call join() or some equivalent to concatenate the values generated by myGenerator without having to create an intermediate array, like the python example above?

Answer 1

The comments above answer your question pretty well. However, I was curious about @jonrsharpe's comment about generating intermediate strings and was wondering how that actually affected performance. So I put a join method on the prototype of a generator and tested it.

Here's the join() code:

function* nGen(start, stop) {
  while (start < stop) {
    yield start
    start++
  }
}

nGen.prototype.join = function(sep) {
  let res = this.next().value
  for (let v = this.next(); !v.done; v = this.next()) {
    res += sep + v.value
  }
  return res
}


let g = nGen(2, 20)

console.log(g.join(','))

The original jsPerf tests in Safari and Chrome showed this working faster than the very common idiom: [...g].join(','). In a JSBench test in 2022, results were inconsistent between Chrome, Firefox, and Edge, but this method is now slower than [...g].join(',') and/or a for loop.

I am far from a master of writing jsPerf/JSBench tests, so maybe I'm screwing something up or misinterpreting the result. But if you're interested: https://jsbench.me/zlkz8tm6vw/1