Promise无法在node.js上解决

Question

I'm having a lot of problems with node.js/Promises at the moment. Currently I'm doing something simple as this:

module.js

var fs = require('fs');

function myFunction() {
    var files = []; 
    for (var i = 0; i < 100; ++i) {
        files.push(fs.writeFile("file-" + i + ".txt", "file" + i, "utf-8", function(err) {
            if (err)
                throw err;
        }));
    }
    Promise.all(files).then(function() {
        return new Promise(function(resolve, reject) {
            resolve('ok');
        })
    });
}
exports.myFunction = myFunction;

main.js

var test = require('./module.js');

test.myFunction().then(function(result) {
    console.log(result);
})

Yet, my output if I run my main.js is:

module.myFunction().then(function(result) {
                   ^

TypeError: Cannot read property 'then' of undefined

I don't know why my module is returning a Promise as undefined. Can anybody help me out here? I just can't wrap my head around this. Thank you very much!

And, while writeFile() writes the files as file-0.txt and so on, the files have no content at all.

Answer 1

You code has two problems:

• the native fs.writeFile() does not return a promise
• your function does not return a promise

For the first problem you can either code some wrapper yourself or use something like fs-promise . For the second problem you really need your function to return a promise for all the files.

Then your code could look like this:

var fsp = require('fs-promise');

function myFunction() {
  var files = []; 
  for (var i = 0; i < 100; ++i) {
    files.push( fsp.writeFile("file-" + i + ".txt", "file" + i, "utf-8" ) );
  }

  return Promise.all( files );

}
exports.myFunction = myFunction;