simple iterator(generator) example

const log = console.log;

const isIterable = a => !!(a && a[Symbol.iterator]);

function *flatten(iter) {

for(const a of iter) {

// if(!isIterable(a)) for(const b of a) yield b;

if(!isIterable(a)) yield* b;

else yield a;

}

}

function *map(f, iter) {

for(const a of iter) yield f(a);

}

const flatMap = (f, iter) => flatten(map(f,iter));

log([...flatten([[1,2], [3,4]])]);

// [1,2,3,4]

log([...flatMap(a => [a,a], [1,2,3])]);

// [1,1,2,2,3,3]