Simple utility for creating variadic intervals

setIntervalTimeout.js

// setIntervalTimeout sits somewhere between setTimeout and setInterval,
// allowing a finite or infinite number of fixed or variadic intervals.

// Returns a cleanup function to clear the current timeout.

// Note: IE does not support generators.

function setIntervalTimeout(callback, iterable, ...args){
  if (!(Symbol.iterator in iterable)){
    throw new Error(`Expected valid iterable. Received: ${iterable}.`);
  }
  
  let timeout = null;
  
  function iterate(itr){
    const next = itr.next();
    if (!next.done){
      timeout = setTimeout(() => {
        callback(...args);
        iterate(itr);
      }, next.value);
    }
  }
  
  iterate(iterable[Symbol.iterator]());
  return () => clearTimeout(timeout);
}

// Bad:
setIntervalTimeout(console.log, { time: 1000 }, "Hello, world!"); // objects are not iterables
setIntervalTimeout(console.log, 1000, "Hello, world!"); // numbers are not iterables, use setTimeout instead.

// Weird:
// strings are iterable and valid timeout values. Will execute once, immediately, for each char.
setIntervalTimeout(console.log, "foobar", "Hello, world!"); 
// arrays are iterable but values are invalid timeout values. Will execute once, immediately, for each element.
setIntervalTimeout(console.log, [{foo: "foo"}], "Hello, world!"); 

// Unlikely - Maps, Sets, WeakMaps, etc:
setIntervalTimeout(console.log, new Map([1000, 1000]), "Hello, world!");
setIntervalTimeout(console.log, new Set([1000, 1000]), "Hello, world!");
setIntervalTimeout(console.log, new WeakMap([1000, 1000]), "Hello, world!");

// Arrays:
// Most of the time you will use an array of numbers.
setIntervalTimeout(console.log, [500], "Hello, world!"); // identical to setTimeout
setIntervalTimeout(console.log, [500, 500], "Hello, world!"); // identical to cancelling setInterval after two fixed intervals.
setIntervalTimeout(console.log, [500, 750, 1000], "Hello, world!"); // Three custom intervals.

// ...but you can technically use strings since JS will coerce them to numbers but don't do this.
setIntervalTimeout(console.log, ["500", "500"], "Hello, world!");

// Iterators:
// Useful for creating intervals programatically
const exponentialBackoff = function* generateFib(){
  let a = 0, b = 1;
  while (Number.isSafeInteger(a + b)){
    const c = a + b;
    a = b, b = c;
    yield c;
  }
}() //intervals increase exponentially - useful when retrying a failed network request

const linearBackoff = function* generateLinear(ms, start = 0){
  let next = start;
  while (Number.isSafeInteger(next + ms)) {
    next += ms;
    yield next;
  }
}(500); // intervals increase linearly - useful for less aggressive backoffs

setIntervalTimeout(console.log, exponentialBackoff, "Retrying IMMEDIATELY RIGHT NOW...");
setIntervalTimeout(console.log, linearBackoff, "Retrying at a leisurely pace...");