Merge sorted iterators

.js

'use strict';
function* merge(iterable1, iterable2) {
	let iter1 = iterable1[Symbol.iterator]();
	let iter2 = iterable2[Symbol.iterator]();
	let a = iter1.next();
	let b = iter2.next();
	let isFirst; 
	
	while(!a.done && !b.done) {
		if (a.value <= b.value) {			
			yield a.value;
			isFirst = true;
		}
		else {
			yield b.value;
			isFirst = false;
		}
		
		if (isFirst) {
			a = iter1.next();
		}
		else {
			b = iter2.next();
		}
	}
	
	if (!a.done) {
		yield a.value;
		yield* iter1;
	}
	if (!b.done) {
		yield b.value;
		yield* iter2;
	}
	
}

console.log(...merge([1, 3, 9], [2, 5, 11, 15])); //1 2 3 5 9 11 15