Error detected unexpectedly

[jansesun]

Recently, I encountered a weird problem when I wrapped fs.exists as a deferred.promisify. My codes as below:

var fs = require('fs'),
    deferred = require('deferred'),
    promisify = deferred.promisify,
    exists = promisify(fs.exists);
exists('')(function(exist) {
    console.log(exist);
});

Once my codes ran, an error that "false" is not an error object is thrown. I was very confused, and then dived into how the exception happened. I find that when the fs.exists is executed normally, it will pass false rather than null to the parameter error. Meanwhile, the function applyFn in 'deferred/ext/function/call-ansyc.js' detects errors by error != null. false != null values true, so the logic goes to the unexpected branch.

[medikoo]

@jansesun fs.exists is not a node callback style function, and deferred.promisify is strictly reserved for functions which obey this contract.

fs.exists is weird (and only) deviation in node.js API (originally it was path.exists), and for this reason it is already deprecated and it's usage is discouraged. Rely on fs.lstat instead.