ES6 Dyanmic Promise Chaining from array

Scenario

I have an array of URLs that I need to download, however each must also be supplied with a unique transaction ID that must be requested from the server and only increments when a request is successful.

Problem

As I loop through the array I need to wait for both the request for the transaction ID and the request for the file to complete before starting the next iteration of the loop but the number of files is not fixed so need to dynamically build a chain of promises.

Pseudocode

Below is some pseudocode, getFiles() is the problem because all the requests get the same transaction Id as they don't wait for the previous request to finish.

function getTransationId(){
    return new Promise((resolve,reject)=> {
        let id = getNextTransactionId();
        if(id!=error){
            resolve(id);
        }else{
            reject(error);
        }
    })
}

function getFile(url, transactionId){
    return new Promise((resolve,reject)=>{
        http.request(url+transactionId, function(err,response){
            if(err){
                reject(err);
            }else{
                resolve(response);
            }
        });
    });
}

function getFilesFromArray(urlArray){
    for(let url of urlArray){
        getTransactionId().then(resolve=>getFile(url,resolve),reject=>console.error(reject));
    }
}

Question

How do I chain chain promises together dynamically?

via Simon