//mergePromise
//data

const timeout = ms => new Promise((resolve, reject) => {
    setTimeout(() => {
        resolve();
    }, ms);
});

const ajax1 = () => timeout(2000).then(() => {
    console.log("1");
    return 1;
});

const ajax2 = () => timeout(1000).then(() => {
    console.log("2");
    return 2;
});

const ajax3 = () => timeout(2000).then(() => {
    console.log("3");
    return 3;
});


function mergePromise(ajaxArray){
    let arr = [];
    let p = Promise.resolve();
    ajaxArray.forEach(item=>{
        p = p.then(data=>{
            if(data){
                arr.push(data);
            }
            return item();
        });
    })
    return p.then(data=>{
        arr.push(data);
        return arr;
    })
}

mergePromise([ajax1, ajax2, ajax3]).then(data => {
    console.log("done");
    console.log(data); // data  [1, 2, 3]
});

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

what is the principle of this mergePromise?
every time I rewrite p? Promise.resolve?