exception handling, thrown errors, within promises

Question

I am running external code as a 3rd party extension to a node.js service. The API methods return promises. A resolved promise means the action was carried out successfully, a f

Answer 1

It is almost the most important feature of promises. If it wasn't there, you might as well use callbacks:

var fs = require("fs");

fs.readFile("myfile.json", function(err, contents) {
    if( err ) {
        console.error("Cannot read file");
    }
    else {
        try {
            var result = JSON.parse(contents);
            console.log(result);
        }
        catch(e) {
            console.error("Invalid json");
        }
    }

});

(Before you say that JSON.parse is the only thing that throws in js, did you know that even coercing a variable to a number e.g. +a can throw a TypeError?

However, the above code can be expressed much more clearly with promises because there is just one exception channel instead of 2:

var Promise = require("bluebird");
var readFile = Promise.promisify(require("fs").readFile);

readFile("myfile.json").then(JSON.parse).then(function(result){
    console.log(result);
}).catch(SyntaxError, function(e){
    console.error("Invalid json");
}).catch(function(e){
    console.error("Cannot read file");
});

Note that catch is sugar for .then(null, fn). If you understand how the exception flow works you will see it is kinda of an anti-pattern to generally use .then(fnSuccess, fnFail).

The point is not at all to do .then(success, fail) over , function(fail, success) (I.E. it is not an alternative way to attach your callbacks) but make written code look almost the same as it would look when writing synchronous code:

try {
    var result = JSON.parse(readFileSync("myjson.json"));
    console.log(result);
}
catch(SyntaxError e) {
    console.error("Invalid json");
}
catch(Error e) {
    console.error("Cannot read file");
}

(The sync code will actually be uglier in reality because javascript doesn't have typed catches)