Introducing Props2Js

One of my principles of Maintainable JavaScript1 is to separate your configuration data from your application logic. Configuration data is hardcoded information that your JavaScript uses to work properly. This could be anything such as a URL or a UI string. For example:

function validate(value) {
    if (!value) {
        alert("Invalid value");
        location.href = "/errors/invalid.php";
function toggleSelected(element) {
    if (hasClass(element, "selected")) {
        removeClass(element, "selected");
    } else {
        addClass(element, "selected");

There are three pieces of configuration data in this code. The first is the string, “Invalid value”, which is displayed to the user. As a UI string, there’s a high chance that it will change frequently. The second is the URL “/errors/invalid.php”. URLs tend to change as development progresses due to architectural decisions. The third is the CSS class name “selected”. This class name is used three times, meaning that a class name change requires changes in three different places, increasing the likelihood that one will be missed.

Configuration data is best extracted from the core application logic, such as:

//Configuration data externalized
var config = {
    MSG_INVALID_VALUE:  "Invalid value",
    URL_INVALID:        "/errors/invalid.php",
    CSS_SELECTED:       "selected"

function validate(value) {
    if (!value) {
        location.href = config.URL_INVALID;

function toggleSelected(element) {
    if (hasClass(element, config.CSS_SELECTED)) {
        removeClass(element, config.CSS_SELECTED);
    } else {
        addClass(element, config.CSS_SELECTED);

This example stores all of the configuration data in the config object. Each property of config holds a single piece of data, and each property name has a prefix indicating the type of data (MSG for a UI message, URL for a URL, and CSS for a class name). The naming convention is, of course, a matter of preference. The important part of this code is that all of the configuration data has been removed from the functions, replaced with placeholders from the config object.

Externalizing the configuration data means that anyone can go in and make a change without fear of introducing an error in the application logic. It also means that the entire config object can be moved into its own file, so edits are made far away from the code that uses the data.

Having an external object managing your configuration data is a good start, but I’m not a fan of storing configuration data directly in JavaScript code. Because such data changes frequently, I prefer to keep it in a simpler file format – one that’s free from worries about missing a semicolon or comma. And that’s when I turned to the Java properties file2.

Java properties files are incredibly simple. One name-value pair per line and comments begin with a #. It’s really hard to mess up this format. Here’s what the previous example’s configuration data looks like in a Java properties file:

# UI Strings
MSG_INVALID_VALUE = Invalid value

# URLs
URL_INVALID = /errors/invalid.php

# CSS Classes
CSS_SELECTED = selected

Even though I had my configuration data in a Java properties file, I had no easy way of making this data available to JavaScript.

This is why I created Props2Js3, a simple tool that does just one thing: reads a Java properties file and outputs it in a format that JavaScript can use. Actually, it’s capable of outputting the data into three formats that JavaScript can use: JSON, JSONP, and regular JavaScript.

java -jar props2js-0.1.0.jar --to jsonp --name myfunc --output result.js

The --to option specifies the output format, either “js”, “json”, or “jsonp”. The --name option specifies either the variable name (for “js”) or the function name (for “jsonp”); this option is ignored for “json”. The --output option specifies the file to write the data into. So this line takes the Java properties file named and outputs JSONP with a callback function of myfunc to a file named result.js.

Props2Js outputs the properties file mentioned above into JSON format:

{"MSG_INVALID_VALUE":"Invalid value","URL_INVALID":"/errors/invalid.php",

Here’s the JSONP output:

myfunc({"MSG_INVALID_VALUE":"Invalid value","URL_INVALID":"/errors/invalid.php",

And here’s the plain JavaScript option with --name config:

var config={"MSG_INVALID_VALUE":"Invalid value","URL_INVALID":"/errors/invalid.php",

Props2Js is also smart enough to know that you’re assigning to an object property if you include a dot in in the --name option. In that case, it omits the var.

Props2Js is available under an MIT License and is hosted at GitHub3.

  1. Maintainable JavaScript 2011 by Nicholas C. Zakas
  2. .properties by Wikipedia
  3. Props2Js

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