#ifndef in javascript

I am looking for a solution to only define a function once in Javascript using something exactly like #ifndef in compiled languages. I found a couple of libraries that were supposed to mimic this functionality but they didn't work.

I am working with MVC 3 Razor and have defined some html helpers do put what are essentially user controls onto the page.

Each control has a set of javascript functions that define specific functionality for that control, so herein lies the issue: the functions get defined multiple times when the helper is called multiple times on a single page.

I am hoping to find a way to keep the very small amount of javascript defined within the helper and not have to divide all of the javascript for each of these small helpers in a separate file.


@helper CmsImage(int id)
  var Html = ((System.Web.Mvc.WebViewPage)WebPageContext.Current.Page).Html;

    <input type="button" class="editor_function" style="display: none;" onclick="editImage(@id); return false;" />
        function editImage(id) {
            $('#alt_text' + id).attr('value', $('#' + id).attr('alt'));
            $('#image_url' + id).attr('value', $('#' + id).attr('src'));

        function saveImage(button, id) {
            $(button).parent().siblings('div.widget-content').html('<img alt="' + $('#alt_text' + id).val() + '" src="' + $('#image_url' + id).val() + '" id="' + id + '" />');
    Image Url:
    <input type="text" id="[email protected]{id.ToString();}" /><br />
    Alt Text:
    <input type="text" id="[email protected]{id.ToString();}" /><br />
    <input type="button" value="save" onclick="saveImage(this, @{id.ToString();});" />

The above doesn't work in the browser, if gives me the error: '48: Unrecognized token ILLEGAL'



As I presume you know, Javascript doesn't have preprocessor directives like C/C++, but you can use regular if statements that are evaluated at run-time like this:

if (typeof myFunc === "undefined") {
    var myFunc = function(a,b) {
        // body of your function here

or for a whole library of functions:

if (!window.controlUtilsDefined) {
    window.controlUtilsDefined = true;

    // put control library functions here

    function aaa() {
        // body here

    function bbb() {
        // body here


or if you want to check based on some other variable:

var myFunc;
if (debugMode) {
    myFunc = function(a,b) {
        // body of your function here
} else {
    myFunc = function(a,b) {
        // body of your alternate function here

If your concern is just that you have multiple copies of the exact same function names in the library that each control uses, that isn't technically a problem in Javascript. The last defined one will be the operative one, but if they're all the same that isn't technically a problem. Only one definition will exist in memory as the later definitions will replace the earlier ones.

If you control the source of the controls, then it would be better to break the common utilities out separately into their own JS file and have the host page just include that utilities script file once.

Or (with a little more work but no additional responsibilities for the host page), each control could dynamically load their utlities from an external JS file and check a known global variable to see if some other control has already loaded the common external JS.


If you have some build script I suggest to use GPP preprocessor(http://en.nothingisreal.com/wiki/GPP, or win version http://makc.googlecode.com/svn/trunk/gpp.2.24-windows/)

So you need to do the following steps:

  1. gpp -o _script.js script.js (where _script.js - your source file with preprocessor commands)
  2. (OPTIONAL) minify script.js (using google closure compiler, etc.)
  3. deploy script.js to your web folder.

In this case you'll get the most optimized js code. And you do not need runtime checks

#define debugMode
#ifdef debugMode
    var myFunc = function(a,b) {
        // body of your function here
    var myFunc = function(a,b) {
        // body of your alternate function here

I see that that the answer provided by jfriend is a bit old when node.js is not around. Pls. check the latest preprocessor.js (available through npm install).

You can use the static conditions like below (from documentation)

 // #ifdef FULL
console.log("Including extension");
// #include "path/to/extension.js"
// #else
console.log("Not including extension");
// #endif

Usage is :

    Usage: preprocess sourceFile [baseDirectory] [-myKey[=myValue], ...] [> outFile]

    preprocess Source.js . -FULL=true > Source.full.js


This is an old question but the answers are now a bit out of date. If you are trying to get better inlining or eliminate dead code you can use google's closure compiler along with the /** @const */ helper

Here is a completely random (useless except for demo) example of @const helper at work with advanced optimizations:

/** @const */
var G=true

if(G){var d=document}
function dummy(){if(!G){var d=document}return d.getElementsByTagName("body")[0]}
function hello(name) {
alert(dummy() + name + dummy());
hello('New user');

compiles to: alert((void 0).getElementsByTagName("body")[0]+"New user"+(void 0).getElementsByTagName("body")[0]);

or without @const:var a=!0;function b(){if(!a)var c=document;return c.getElementsByTagName("body")[0]}alert(b()+"New user"+b());

The benefit of this is that the code will still work during development without having to preprocess it before each run. (with the added benefit of better optimization)


Old theme, but for anyone interested in the required solution I wrote jspreproc to deploy some riot modules, all in JavaScript for node 0.10.0 and above.

jspreproc is open source, remove empty lines, supports filters for preserve comments of different types, and conditional comments in C-style:

  • #if, #elif, #else, #endif with expression and defined() support
  • #define with expression and basic macro-replacement
  • #ifdef, #ifndef
  • #include, #include_once

Installation and usage example:

$ npm -g i jspreproc
$ jspp -D RELEASE --empty-lines 0 lib/file1.js lib/file2.js > dist/app.js

Read more at the jspreproc repository on github.


In modern times, it looks like a job for RequireJS or equivalent AMD module. All of the other answers talk about precompilation and minification - which are good, and should be considered in addition.


Its not possible to use Preprocessor or #define function inside JavaScript... JavaScript is loosly coupled and interpreted language..... But you can use this in Typescript as it is Strongly coupled and uses compiler...


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