lazy evaluation

advanced-generator-functions.js

let random = function*() {
 
    while(true) {
        console.log("make random")
        yield Math.random();
    }
}

let filter = function*(items, predicate) {
    for(let item of items){
        console.log("filter", item);
        if(predicate(item)){
            yield item;
        }
    }
};


let take = function*(items, number) {
    let count = 0;
    if(number < 1) return;
 
    for(let item of items){
        console.log("take", item);
        yield item;
         
        count += 1;
        if(count >= number) {
            console.log('finished');
            return;
        }
    }
};

let result = take(filter(random(), n => n < 0.5), 4);

// console.log(Array.from(result));

//Array.from is equivalent to for of

for( item of result ) {
    console.log('item: ', item);
}

for( item of result ) {
    console.log('item: ', item);  // no more vaules available 
}


// console.log(Array.(result).length);

An interesting thing is that 'result' variable will be empty array after you use the variable once in loop