fs.exists doesn't promisify into a good fs.existsAsync

Maybe should be mentioned in documentation.

Problem with promisifyAll of Node’s fs for method fs.existsAsync.

Obvious reason: fs.exists doesn’t callback with an err for first argument, just the result.

Workaround for many uses: Use fs.stat instead.

Example code to illustrate problem:

var Promise = require('bluebird');
var fs = require('fs');
Promise.promisifyAll(fs);

function promise(path) {
    fs.exists(path, function(exists) {
        console.log("callback with path " + path + " exists " + exists);
    });
    fs.stat(path, function(err, stats) {
        if (err) {
            console.log("stat callback with path " + path + " err " + err);
        } else {
            console.log("stat callback with path " + path + " file " + stats.isFile() + ", directory " + stats.isDirectory());
        }
    });
    return fs.existsAsync(path)
    .then(function resolve(exists) {
        console.log("promise with path " + path + " resolves exists " + exists);
    },function reject(reason) {
        console.log("promise with path " + path + " rejects reason " + reason);
    });
}

Promise.try(promise,'/');
Promise.try(promise,'/tmp');
Promise.try(promise,'/somethingimprobable');

with bluebird 2.5.0 (and 2.4.3) produces output

callback with path / exists true
promise with path / rejects reason OperationalError: true
callback with path /tmp exists true
promise with path /tmp rejects reason OperationalError: true
callback with path /somethingimprobable exists false
promise with path /somethingimprobable resolves exists undefined

which means result true gets wrapped as an error.

[srguiwiz]

Better workaround:

fs.existsAsync = Promise.promisify
(function exists2(path, exists2callback) {
    fs.exists(path, function callbackWrapper(exists) { exists2callback(null, exists); });
 });

Example code proving it works:

var Promise = require('bluebird');
var fs = require('fs');
Promise.promisifyAll(fs);

fs.existsAsync = Promise.promisify
(function exists2(path, exists2callback) {
    fs.exists(path, function callbackWrapper(exists) { exists2callback(null, exists); });
 });

function promise(path) {
    fs.exists(path, function(exists) {
        console.log("exists callback with path " + path + " exists " + exists);
    });
    return fs.existsAsync(path)
    .then(function resolve(exists) {
        console.log("promise with path " + path + " resolves exists " + exists);
    },function reject(reason) {
        console.log("promise with path " + path + " rejects reason " + reason);
    });
}

Promise.try(promise,'/');
Promise.try(promise,'/tmp');
Promise.try(promise,'/somethingimprobable');

producing output:

exists callback with path / exists true
promise with path / resolves exists true
exists callback with path /tmp exists true
promise with path /tmp resolves exists true
exists callback with path /somethingimprobable exists false
promise with path /somethingimprobable resolves exists false

[petkaantonov]

FWIW, there should be no reason to use exists, after you have checked that the file exists, the file may disappear before you start to read it and you need the same error handling as you would have needed if you just didn’t use exists in the first place.