error-handling-promises.js

// Initialization
var log = "";
 
function doWork() {
    log += "W";
    return Promise.resolve();
}
 
function doError() {
    log += "E";
    throw new Error("oops!");
}
 
function errorHandler(error) {
    log += "H";
}

// snippet 1
doWork()
    .then(doWork)
    .then(doError)
    .then(doWork)  // this will be skipped
    .then(doWork)   // this will be skipped
    .catch(errorHandler)
// 'log' variable contains: "WWEH"

// snippet 2
doWork()
    .then(doWork)
    .then(doError)
    .then(doWork) // this will be skipped
    .then(doWork, errorHandler)  // doWork will be skipped
// 'log' variable contains: "WWEH" 

// the basic difference between the above two snippets show up when we do the following:

// rewriting snippet 1
doWork()
    .then(doWork)
    .then(doWork) 
    .then(doWork)
    .then(doError)
    .catch(errorHandler)
// 'log' variable contains: "WWWWEH"

// rewriting snippet 2
doWork()
    .then(doWork)
    .then(doWork)
    .then(doWork)
    .then(doError, errorHandler)    // errorHandler is skipped
    .catch(errorHandler)            // will be caught here (instead of above)
// 'log' variable contains: "WWWWEH"