承诺链未按预期顺序执行-Node.js

Question

I have a chain of 4 promises, and 1 function at the end. The final function is executing before the previous promises in the chain have resolved.

Can someone explain to me why this might be happening?

Here is the promise chain:

 updateGdax(db) .then(updateBitstamp(db)) .then(updateBitfinex(db)) .then(updatePoloniex(db)) .then(coinMarketData.updateCoinMarketData(db)) .then(addRates(db)); //this function is executing after the first promise in the chain. 

I would like each function to execute after the one listed before it, so addRates(db) should be executed last.

I can post the code from the promise functions if needed for further analyses, but I really just want to understand WHY this would happen, as my understanding is that functions in a promise chain won't execute unless the previous promise in the chain has resolved.

Answer 1

Unless those update functions in the then calls are partially applied (unless they return a function), they are being executed before the then is called. You need to wrap them in an anonymous function to have them executed in order. Do what the other answer says or use fat arrows:

updateGdax(db)
  .then(()=>updateBitstamp(db))
  .then(()=>updateBitfinex(db))
  .then(()=>updatePoloniex(db))
  .then(()=>coinMarketData.updateCoinMarketData(db))
  .then(()=>addRates(db)); 

If your update functions could be rewritten to return the db after completing, then you could rewrite the calls like so, point free style:

updateGdax(db)
  .then(updateBitstamp)
  .then(updateBitfinex)
  .then(updatePoloniex)
  .then(coinMarketData.updateCoinMarketData)
  .then(addRates); 

Each function, would then look something like this:

function updateGdax(db) {
   return db.doSomething().then(()=> db)
}

Follow that pattern, and you have yourself some nice looking javascript.

Answer 2

And have a look at the new async/await, included in nodejs 8. It is much more intuitive:

async function main() {
  await updateGdax(db)
  await updateBitstamp(db)
  await updateBitfinex(db)
  await updatePoloniex(db)
  await coinMarketData.updateCoinMarketData(db)
  await addRates(db)
}

main().catch(e => console.error(e))

Answer 3

Try below approach,

updateGdax(db)
  .then(function(){
    return updateBitstamp(db)
  }).then(function (){
    return updateBitfinex(db);
  }).then(function() {
    return updatePoloniex(db);
  }).then(function(){
    return coinMarketData.updateCoinMarketData(db)
  }).then(function(){
    return addRates(db);
  }).catch(function(err){
    console.log(err);
  });

Hope this will work. If any of the function is returning any value and if you want to use it in subsequent function the pass that value in following function() used inside then. Refer : https://strongloop.com/strongblog/promises-in-node-js-an-alternative-to-callbacks/