Manually promisifying a callback

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Sometimes it might be necessary to manually promisify a callback function. This could be for a case where the callback does not follow the standard error-first format or if additional logic is needed to promisify:

Example with fs.exists(path, callback):

var fs = require('fs');

var existsAsync = function(path) {
  return new Promise(function(resolve, reject) {
    fs.exists(path, function(exists) {
      // exists is a boolean
      if (exists) {
        // Resolve successfully
        resolve();
      } else {
        // Reject with error
        reject(new Error('path does not exist'));
      }
    });
});

// Use as a promise now
existsAsync('/path/to/some/file').then(function() {
  console.log('file exists!');
}).catch(function(err) {
  // file does not exist
  console.error(err);
});

Found a mistake? Have a question or improvement idea? Let me know.

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