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JavaScript Asynchronous Programming with JavaScript Understanding Promises Promises Review

But with .then() you can't pass values as arguments into callbacks like you can with nested callbacks?

See the following code where I've written nested callbacks with continuation-passing style as an alternative to the chained mathPromise.then() methods in the instruction.

function calculate(x, callback) {
  callback(x);
}
function add(x, y, callback) {
  callback(x + y)
}
function finalValue(nextValue) {
  console.log("The final value is ${nextValue}")
}

// Using nested callbacks to pass n
calculate(5, (n) => { // calculate passes callback with n param executing add()
  // n param becomes input x and gets added 10
  add(n, 10, (n) => { // add passes callback with n param executing subtract()
    subtract(n, 2, (n) => {
      multiply(n, 5, (n) => {
        finalValue(n) ; 
      });
    });
  });
});

1 Answer

Steven Parker
Steven Parker
229,581 Points

Note that your example is not an alternative to the lesson code as it contains no promises or async operations.

But you can pass values using promises and then by currying the callbacks to include the extra arguments:

const add =      (x, y) => x + y;
const subtract = (x, y) => y - x;
const multiply = (x, y) => x * y;
const finalValue = nextValue => console.log(`The final value is ${nextValue}`);

const value = 5;
const mathPromise = new Promise( resolve => { resolve(value); } );

mathPromise
  .then(add.bind(this, 10))
  .then(subtract.bind(this, 2))
  .then(multiply.bind(this, 5))
  .then(finalValue);

// The final value is 65

Fascinating!