promise chain example

promise-chain.js

// thanks to @kakashiAL from #node.js on freenode

function getDataA(data) {
  return new Promise (function(res,rej) {
    res(data);
  })
}

function getDataB(data) {
  return new Promise (function(res,rej) {
    var x = [data,data,data]
    console.log(x);
    res(x);
  })
}

function getDataC(data) {
  return new Promise (function(res,rej) {
    var x = data[0];
    console.log(x);
    res(x);
  })
}

function getDataD(data) {
  return new Promise (function(res,rej) {
    var x = data + data;
    console.log(x);
    res(x);
  })
}



// getDataA is a function that gets data from a database/network/file
// it returns a promise
// everything what returns a promise has the method then() and catch()
getDataA('Hi')
  .then(dataA => {
    // getDataB also returns a promise
    // it data is only available in the next then function
    // we have to return it to be able to get its data in the next then()
    return getDataB(dataA);
  }).then(dataB => {
    return getDataC(dataB);
  }).then(dataC => {
    return getDataD(dataC);
  }).catch(error => {
    // ERROR HANDLING HERE
    console.log("you have an error: ", error);
  });