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Safety of then - thrown errors

We need to write a function that reads a config file and parses it as JSON. How can we do that safely?


With callbacks we will need to wrap JSON.parse in a try catch block because it throws errors:

function readConfig(file, callback) {
    fs.readFile(file, function(err, content) {
        if (err) return callback(err);
        var parsed;
        try { parsed = JSON.parse(content.toString()) }
        catch (error) { return callback(err); }
        callback(null, parsed);


With promises no special treatment is required:

function readConfig(file) {
    return fs.readFile(file).then(function(res) {
        return JSON.parse(content.toString());


In the callback example, we must be careful not to wrap the callback inside the try block! Otherwise, an error thrown from inside that callback will cause the catch block to call it again.

In the promises example, any errors thrown inside then will be propagated as errors with the promise, and we can handle them in the error handler of that promise:

readConfig(file).done(function(config) {
    // Got configuration.
}, function(error) {
    // JSON parse error or file read error.