Moving from when.js to ES6 Promises

I've been using when.js for promises in node.js. I have a function like the following:

function my_func() {
    var d = when.defer();

    // Async actions simulated with timeout
    setTimeout(function() {
        //...
        if(error) {
            return d.resolve(error);
        }
        d.resolve(result);
    }, 1000)

    return d.promise;
}

Which I would call like so:

my_func().then(
    function(res) {
        //...
    },
    function(err) {
        // Handle error
    }
);

How do I do the same thing with ES6 Promises?

Your original function had some mistakes in the if (error) condition, so here's the updated snippet:

function my_func() {
    var d = when.defer();

    // Async actions simulated with timeout
    setTimeout(function() {
        //...
        if(error) {
            d.reject(error);
        }
        d.resolve(result);
    }, 1000)

    return d.promise;
}

which turns into

function my_func() {
  return new Promise(function (resolve, reject) {
    //Async actions simulated with timeout
    setTimeout(function () {
      //...
      if (error) {
        reject(error);
      }
      resolve(result);
    }, 1000);
  });
}

This is covered reasonably well in MDN's Promise documentation