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JavaScript Promises
The onFulfilled and onRejected Functions

To handle a “successful” promise, or a promise that resolved, we invoke .then() on the promise, passing in a success handler callback function:

const prom = new Promise((resolve, reject) => { resolve('Yay!'); }); const handleSuccess = (resolvedValue) => { console.log(resolvedValue); }; prom.then(handleSuccess); // Prints: 'Yay!'

Let’s break down what’s happening in the example code:

  • prom is a promise which will resolve to 'Yay!'.
  • We define a function, handleSuccess(), which prints the argument passed to it.
  • We invoke prom‘s .then() function passing in our handleSuccess() function.
  • Since prom resolves, handleSuccess() is invoked with prom‘s resolved value, 'Yay', so 'Yay' is logged to the console.

With typical promise consumption, we won’t know whether a promise will resolve or reject, so we’ll need to provide the logic for either case. We can pass both an onFulfilled and onRejected callback to .then().

let prom = new Promise((resolve, reject) => { let num = Math.random(); if (num < .5 ){ resolve('Yay!'); } else { reject('Ohhh noooo!'); } }); const handleSuccess = (resolvedValue) => { console.log(resolvedValue); }; const handleFailure = (rejectionReason) => { console.log(rejectionReason); }; prom.then(handleSuccess, handleFailure);

Let’s break down what’s happening in the example code:

  • prom is a promise which will randomly either resolve with 'Yay!'or reject with 'Ohhh noooo!'.
  • We pass two handler functions to .then(). The first will be invoked with 'Yay!' if the promise resolves, and the second will be invoked with 'Ohhh noooo!' if the promise rejects.

Let’s write some onFulfilled and onRejected functions!



Take a look at the provided code. We require in a function, checkInventory(). It builds on the logic of the orderSunglasses() function you wrote in a previous exercise.

  • checkInventory() takes in an array representing an order and returns a promise.
  • If every item in the order is in stock, that promise resolves with the value "Thank you. Your order was successful."
  • Otherwise, the promise rejects with the value "We're sorry. Your order could not be completed because some items are sold out".

We used setTimeout() to ensure that the checkInventory() promise settles asynchronously.

If you’d like, look at the library.js file to see how it works. Press “Check Work” when you’re ready to move on.


Write a function, handleSuccess(). You’ll use this function later on as your success handler. handleSuccess() should have one parameter, representing a resolved value. Inside the body of handleSuccess(), log the parameter to the console.


Write a function, handleFailure(). You’ll use this function later on as your failure handler. handleFailure() should have one parameter, representing a rejection reason. Inside the body of handleFailure(), log the parameter to the console.


Invoke checkInventory() with order. This will return a promise. Attach a .then() function to this. Pass into .then() the two handlers you wrote as callback functions.


Type node app.js in the terminal and hit enter.

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