SOURCE

const fetch = (url, time) => {
    return function() {
        return new Promise(resolve => {
            setTimeout(() => {
                resolve(url);
            }, time)
        })
    }
}


function fetchLimit(tasks, limit = 2) {
    let count = 0;
    const result = [];
    const len = tasks.length;
    return new Promise((resolve, reject) => {

        function run() {
            if (tasks.length && count < limit) {
                count++;
                const fn = tasks.shift();
                fn().then(item => {
                    count--;
                    result.push(item);
                    if (result.length === len) {
                        resolve(result);
                    } else {
                        run();
                    }
                }).catch(err => {
                    reject(err);
                })
            }
        }

         for (let i = 0; i < tasks.length; i++) {
            run();
        }

    })

}


const tasks = [
    fetch('this-1', 1000),
    fetch('this-2', 2000),
    fetch('this-3', 3100),
    fetch('this-4', 1000),
    fetch('this-5', 1000),
    fetch('this-6', 2000),
    fetch('this-7', 1000),
    fetch('this-8', 1000),
]


fetchLimit(tasks).then(res => {
    console.log('res: ', res);
});