Promises, pass additional parameters to then chain

Question

A promise, just for example:

var P = new Promise(function (resolve, reject) {
  var a = 5;
  if (a) {
    setTimeout(function(){
      resolve(a);
    }, 3000);
  } else {
    reject(a);
  }
});

After we call the .then() method on the promise:

P.then(doWork('text'));

Then doWork function looks like this:

function doWork(data) {
  return function(text) {
    // sample function to console log
    consoleToLog(data);
    consoleToLog(b);
  }
}

How can I avoid returning an inner function in doWork, to get access to data from the promise and text parameters? Are there any tricks to avoiding the inner function?

Answer 1

You can use Function.prototype.bind to create a new function with a value passed to its first argument, like this

P.then(doWork.bind(null, 'text'))

and you can change doWork to,

function doWork(text, data) {
  consoleToLog(data);
}

Now, text will be actually 'text' in doWork and data will be the value resolved by the Promise.

Note: Please make sure that you attach a rejection handler to your promise chain.

---

Working program: Live copy on Babel's REPL

function doWork(text, data) {
  console.log(text + data + text);
}

new Promise(function (resolve, reject) {
    var a = 5;
    if (a) {
      setTimeout(function () {
        resolve(a);
      }, 3000);
    } else {
      reject(a);
    }
  })
  .then(doWork.bind(null, 'text'))
  .catch(console.error);