var a = [1000, 500]
var b = []

a.forEach(e => {
    let c = 0
    b.push(function () {
        return new Promise((resolve) => {
            setTimeout(() => {
                console.log(e)
                c++
                resolve(c)
            }, e);
        }).then((c) => {
            if (b[c] === undefined) return
            b[c]()
        })
    })
})
b[0]()