Promises, pass additional parameters to then chain

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?

Perhaps the most straightforward answer is:

P.then(function(data) { return doWork('text', data); });

Or, since this is tagged ecmascript-6, using arrow functions:

P.then(data => doWork('text', data));

I find this most readable, and not too much to write.

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);

Use currying.

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

var curriedDoWork = function(text) {
    return function(data) {
        console.log(data + text);
    }
};

P.then(curriedDoWork('text'))
.catch(
    //some error handling
);

The new answer to this question is to use arrow functions (which automatically bind the this and are much more readable). Google for links such as: https://2ality.com/2016/02/arrow-functions-vs-bind.html

You can set the text like:

this.text = 'text';
P.then(data => doWork(data));

Note: this.text inside doWork will evaluate to 'text'.

This is suggested by jib above and that (or this!) should be the accepted answer now.

Lodash offers a nice alternative for this exact thing.

 P.then(_.bind(doWork, 'myArgString', _));
 
 //Say the promise was fulfilled with the string 'promiseResults'

 function doWork(text, data) {
     console.log(text + " foo " + data);
     //myArgString foo promiseResults
 }

Or, if you'd like your success function to have only one parameter (the fulfilled promise results), you can utilize it this way:

P.then(_.bind(doWork, {text: 'myArgString'}));

function doWork(data) {
    console.log(data + " foo " + this.text);
    //promiseResults foo myArgString
}

This will attach text: 'myArgString' to the this context within the function.

use this so you can access global variable inside the promise body

var ref=this;

Example

p.then((data)=>{
  var ref=this;
});