callback-loader

Webpack loader that parses your JS, calls specified functions and replaces their with the results.

Installation

npm install callback-loader --save-dev

Usage

Source file:

var a = multBy2(10);
var b = mult(10, 3);
var c = concat("foo", "bar");

Webpack config:

{
    ...
    callbackLoader: {
        multBy2: function(num) {
            return num * 2;
        },
        mult: function(num1, num2) {
            return num1 * num2;
        },
        concat: function(str1, str2) {
            return '"' + str1 + str2 + '"';
        }
    }
}

Result:

var a = 20;
var b = 30;
var c = "foobar";

Notice that quotes was added in concat function.

Loader parameters

You can choose which functions will be processed in your query:

'callback?mult&multBy2!example.js'

Result for this query will be this:

var a = 20;
var b = 30;
var c = concat("foo", "bar");

Different configs

Webpack config:

{
    ...
    callbackLoader: {
        concat: function(str1, str2) {
            return '"' + str1 + str2 + '"';
        }
    },
    anotherConfig: {
        concat: function(str1, str2) {
            return '"' + str1 + str2 + '-version2"';
        }
    }
}

Loader query:

'callback?config=anotherConfig!example.js'

Result for this query will be this:

var a = multBy2(10);
var b = mult(10, 3);
var c = "foobar-version2";

Cache

The loader is cacheable by default, but you can disable cache if you want:

'callback?cacheable=false!example.js'

Restrictions

• No expressions and variables in function arguments yet, sorry. Only raw values.
• No async callbacks yet.

Use cases

• Build time cache.
• Using node modules.
• Localization (see example below)
• Using any other build-time stuff (compiler directives, version number, parameters, etc)

Real life examples

Localization

Let's say we have two language versions and we want to use messages for both of them in a same place, like this:

showMessage(localize{en: 'Hello, world!', ru: 'Привет, мир!'});

But in this case we should require localize function everywhere. Besides it is an redundant call and excess result code size.

So let's just move all the localize calls to build time!

Webpack config:

var languages = ['en', 'ru'];

module.exports = languages.map(function (language) {
    return {
        ...
        output: {
            filename: '[name].' + language + '.js'
        },
        callbackLoader: {
            localize: function (textObj) {
                return '"' + textObj[language] + '"';
            }
        }
    }
});

That's all! Now take a look at our localized code.

bundle.en.js:

showMessage("Hello, world!");

bundle.ru.js

showMessage("Привет, мир!");

Using API at build time

Ok, let's say we need an array of points for a map in points.js file:

module.exports = [
    {
        name: 'Moscow',
        coords: [37.617, 55.756]
    }, {
        name: 'Tokyo',
        coords: [139.692, 35.689]
    }, ...
]

But we don't want to search and write coordinates by yourself. We just want to type city names and let Google Maps API do the REST. But at the same time it's not a good idea to send hundreds of requests each time user open your map. Can we do this once in a build time?

Let's write something like this:

module.exports = [
    {
        name: 'Moscow',
        coords: okGoogleGiveMeTheCoords('Moscow, Russia')
    }, {
        name: 'Tokyo',
        coords: okGoogleGiveMeTheCoords('Tokyo, Japan')
    }, ...
]

Looks much more pretty, right? Now we just need to implement okGoogleGiveMeTheCoords and config callback-loader:

var request = require('sync-request');
...
var webpackConfig = {
    ...
    pointsCallback: {
        okGoogleGiveMeTheCoords: function (address) {
            var response = request('GET', 'http://maps.google.com/maps/api/geocode/json?address=' + address + '&sensor=false');
            var data = JSON.parse(response.getBody());
            var coords = data.results[0].geometry.location;
            return '[' + coords.lng + ', ' + coords.lat + ']';
        }
    }
}

Now write a require statement:

var points = require('callback?config=pointsCallback!./points.js');

And in points we have the array from the first example but we didn't write none of coordinates.

Using dynamic formed requires

Webpack only knows about requires which had been written by your hands. But what if we want to write requires by a script? E.g. we want to require all modules from array (in case we need to configure them in external config).

components.js

module.exports = function () {
    requireComponents();
};

webpack config

    callbackLoader: {
        requireComponents: function() {
            var modules = ["menu", "buttons", "forms"];
            return modules.map(function (module) {
                var moduleLink = 'components/' + module + '/index.js';
                return 'require("' + moduleLink + '");';
            }).join('\n');
        }
    }

So if we load our components.js with callback-loader, result will be this:

module.exports = function () {
    require("components/menu/index.js");
    require("components/buttons/index.js");
    require("components/forms/index.js");
};

Now we have to apply this script (using apply-loader which simply adds an execution statement to the module) and all this dependencies will be resolved:

require('apply!callback!./components.js');