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 a success callback and a failure 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.

Note: The success callback is sometimes called the “success handler function” or the onFulfilled function. The failure callback is sometimes called the “failure handler function” or the onRejected function.

Let’s write some success and failure callbacks!