use Promise to implement

function postImg(file){
    return new Promise(res => {
        var formdata = new FormData()
        formdata.append('img', file)
        axios.post(url, formdata)
        .then(result => {res(result)})
    })//promise
}

var fileArr = []
//file
filePromises = fileArr.map(val => postImg(val))
//promise
Promise.all(filePromises)
.then(val => {
    // 
})