Node.js / Promises Call sequence

Question

I have the following node.js code:

var params = initDbParams();
query().then(orders => function(){
  return match(orders);
}).then(() => function(){
  return place();
}).catch(err => {
  console.log("Error");
});


function initDbParams() {
  console.log("initDbParams Start");
  var a = 1;
  var b = 2;
  var c = 3;
  console.log("initDbParams End");
  return a+b+c;
}

function update() {
  return new Promise(function(resolve, reject ){
    console.log("update Start");
    setTimeout(function(){
        console.log("update timeout over");
    }, 1000);
    console.log("update End");
    resolve();
  });
}

function del() {
  return new Promise(function(resolve, reject ){
    console.log("delete Start");
    setTimeout(function(){
        console.log("del timeout over");
    }, 1200);
    console.log("delete End");
    resolve();
  });
}

function place() {
  return new Promise(function(resolve, reject ){
    console.log("place Start");
    setTimeout(function(){
        console.log("place timeout over");
    }, 1500);
    console.log("place End");
    resolve();
  });
}

function write() {
  return new Promise(function(resolve, reject ){
    console.log("write Start");
    setTimeout(function(){
        console.log("write timeout over");
    }, 2000);
    console.log("write End");
    resolve();
  });
}

function query() {
  return new Promise(function(resolve, reject ){
    console.log("query Start");
    setTimeout(function(){
        console.log("query timeout over");
    }, 3000);
    var orders = [12,24,25,100];
    console.log("query End");
    resolve(orders);
  });
}

function match(orders) {
  return new Promise(function(resolve, reject ){
    console.log("match Start");
    while(orders.length !== 0)
    {
      var order = orders.shift();
      del().then(() => {
        write();
      })
    }
    console.log("match End");
    resolve();
  });
}

I get the following output:

1. initDbParams Start
2. initDbParams End
3. query Start
4. query End

I would expect the following output:

1. initDbParams Start
2. initDbParams End
3. query Start
4. query timeout over
5. query End
6. match Start
   7. del Start
   8. del End
   9. write Start
   10. write End
7. match End

What do I Need to change to get the expected results? Thanks for your support!

Answer 1

setTimeout is still asynchronous, and being called inside a promise doesn't change that. All your timeout over logs will happen later, while the code immediately continues to log end and resolve the promise. You need to call resolve in the asynchronous callback! Same thing in match, where you should also avoid the Promise constructor antipattern.

It should look like this:

function delay(t) {
  return new Promise(function(resolve) {
    setTimeout(function() {
      console.log("timeout over");
      resolve(); // in here!
    }, t);
  });
}

You can then reuse that function to avoid repeating yourself:

function update() {
  console.log("update Start");
  return delay(1000).then(function(){
    console.log("update End");
  });
}

function del() {
  console.log("delete Start");
  return delay(1200).then(function() {
    console.log("delete End");
  });
}

function place() {
  console.log("place Start");
  return delay(1500).then(function() {
    console.log("place End");
  });
}

function write() {
  console.log("write Start");
  return delay(2000).then(function() {
    console.log("write End");
  });
}

function query() {
  console.log("query Start");
  return delay(3000).then(function() {
    var orders = [12,24,25,100];
    console.log("query End");
    return orders;
  });
}

function match(orders) {
  console.log("match Start");
  var promises = orders.map(function(order) {
    return del().then(write);
  });
  return Promise.all(promises).then(function() {
    console.log("match End");
  });
}

Also, .then(orders => function(){ … }) doesn't make sense. It should be either

query().then(function(orders) {
  return match(orders);
}).then(function() {
  return place();
}).catch(err => {
  console.log("Error");
});

or

query().then(orders => 
  match(orders)
).then(() =>
  place()
).catch(err => {
  console.log("Error");
});

but not both an arrow function and a function expression.