yield.js

// makes a "lazy array" of all even numbers 2...infinity
function * evens(n = 0) {    // "Generator function"
    while (true) {
        n += 2
        yield n;
    }
} 

let allEvens = evens()    // allEvens is a "generator"

// manually get a few
console.log(allEvens.next())
console.log(allEvens.next())

// loop until some random thing cuts us off... but laZily!
for (num of allEvens) {
    console.log(num);
    if (Math.random() < 0.05) break;
}