itertools.tee for JavaScript

tee.js

function tee(iterable, num){
    let internalIterator = iterable[Symbol.iterator]();
    let buffer = [];

    //it's very interesting that a generator function can capture the outer variables in a closure, though
    //in the end what is being generated by the compiler is an object with a next() method, not a function having access to the activation object
    function* generatorFn(){
        let pos = 0;
        let finished = false;
        while (!finished){
            if (pos < buffer.length){
                yield buffer[pos];
            }
            else{
                it = internalIterator.next();
                if (it.done){
                    finished = true;
                }
                else{
                    buffer.push(it.value);
                    yield it.value;
                }
            }
            pos++;
        }
    };
    
    let generatorObjs = []
    for (let i=0; i<num; i++){
        generatorObjs.push(generatorFn());
    }
    return generatorObjs;
}

//let items = ["a", "b", "c"];
let generatorObj = (function*(){
    yield "a";
    yield "b";
    yield "c";
})()

/*can be iterated only once
for (it of generatorObj){
    console.log(it);
}

//here the iterator is already complete, so nothing to iterate
for (it of generatorObj){
    console.log(it);
}
*/


let [iter1, iter2, iter3, iter4] = tee(generatorObj, 4);

console.log(iter1.next().value);

console.log(iter2.next().value);
console.log(iter2.next().value);
console.log(iter2.next().value);
console.log(iter2.next().value);

console.log(iter1.next().value);
console.log(iter1.next().value);
console.log(iter1.next().value);

for (it of iter3){
    console.log(it);
}

for (it of iter4){
    console.log(it);
}
/*
a
a
b
c
undefined
b
c
undefined
a
b
c
a
b
c
*/