HOW to implement this method with a promise

This question already has an answer here:

How do I convert an existing callback API to promises? 17 answers

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In the browser world you live in now, how you create your own promise unfortunately depends upon which promise library you are using. Here are a couple ways you could create a geocode(address) function that returns its result via a promise:

// jQuery promises
function geocode(address) {
    var geocoder = new google.maps.Geocoder();

    var def = $.Deferred();
    geocoder.geocode({ 'address': value }, function (results, status) { // called asynchronously
        if (status == google.maps.GeocoderStatus.OK) {
            def.resolve(results);
        } else {
            def.reject(status);
        }
    });
    return def.promise();
}

// ES6 style (supported by many polyfills, promise libraries and some browsers natively)
// my favorite library is Bluebird which this would work with
function geocode(address) {
    var geocoder = new google.maps.Geocoder();

    return new Promise(function(resolve, reject) {
        geocoder.geocode({ 'address': value }, function (results, status) { // called asynchronously
            if (status == google.maps.GeocoderStatus.OK) {
                resolve(results);
            } else {
                reject(status);
            }
        });
    });
}

Both can be used like this which is nice and simple:

geocode(address).then(function(result) {
    // code here that can use the result
}, function(errStatus) {
    // code here to handle an error
});

Note: you can't take an asynchronous operation like this and make it into a synchronous operation. You just can't do that in Javascript. So, you have to learn how to program with asynchronous operations and promises are one convenient way of doing that.

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Other references:

Wrap Google map geocoder.geocode in a Promise

Promises in the Google APIs JavaScript Client Library

How do I convert an existing callback API to promises?

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One can "wrap" the geocode async call as follows - assuming the return value of the callback itself is irrelevant.

Note that this cannot be used to make the asynchronous call synchronous; but it does "wrap" the asynchronous callback to use a promise.

function geocodeWithPromise(data) {
    // Different frameworks provided different methods of creating and
    // using their promise implementation. Eg in jQuery the following would be:
    //   promise = $.Deferred();
    var promise = makePromise();
    geocoder.geocode(data, function (results, status) {
        if (status == google.maps.GeocoderStatus.OK) {
            promise.resolve(results, status);
        } else {
            promise.reject(status);
        }
    });
    return promise;
}

// The call is "synchronous" and a promise is returned immediately - 
// but done/fail are called later as they are [themselves callbacks and]
// part of the asynchronous promise.
geocodeWithPromise({ 'address': value })
  .done(function () { .. })
  .fail(function () { .. });

The tutorial JavaScript Promises: There and back again explains promises in more detail, and covers ES6-style promises.

(As jfriend keeps pointing out, the above code uses done/fail which are not required - as they can be implemented with then - and are only found in some promise implementations such as jQuery.)

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