reading file with ES6 promises

Question

let arr = [];

function getData(fileName, type) {
    return fs.readFile(fileName,'utf8', (err, data) => {
        if (err) throw err;

        return new Promise(function(resolve, reject) {
            for (let i = 0; i < data.length; i++) {
                arr.push(data[i]);
            }

            resolve();
        });
    });
}

getData('./file.txt', 'sample').then((data) => {
    console.log(data);
});

When I use above code and run it in command line using nodejs I get following error.

getData('./file.txt', 'sample').then((data) => {
                               ^

TypeError: Cannot read property 'then' of undefined

How can I solve this?

Answer 1

You'll want to wrap the entire fs.readFile invocation inside a new Promise, and then reject or resolve the promise depending on the callback result:

function getData(fileName, type) {
  return new Promise(function(resolve, reject){
    fs.readFile(fileName, type, (err, data) => {
        err ? reject(err) : resolve(data);
    });
  });
}

[UPDATE] As of Node.js v10, you can optionally use the built-in Promise implementations of the fs module by using fs.promises.<API>. In the case of our readFile example, we would update our solution to use fs.promises like this:

function getData(fileName, type) {
  return fs.promises.readFile(fileName, {encoding: type});
}