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Webpack loader that allows JavaScript or TypeScript conditional compilation (#if ... #elif ... #else ... #endif) directly from Webpack.

Conditional compilation directives are written inside /// triple slash comment so that they don't effect normal JavaScript or TypeScript parsing.


/// #if DEBUG
console.log("there's a bug!");
/// #endif

The DEBUG or any other variable can be specified when configuring the Webpack loader (see below).

The directive #if accepts any valid JavaScript expression:

/// #if PRODUCTION && version.charAt(0)=='X'
/// #endif

If the expression is true the block of code between #if and #endif is included, otherwise is excluded by commenting it out.

Additionally, #elif and #else clauses can be added to an #if clause:

/// #if env == 'PRODUCTION'
/// #elif env == 'DEBUG'
/// #else
console.log('Something else!');
/// #endif

The #if clauses can also be nested:

      /// #if OS=="android"
      /// #elif OS=="ios"
      /// #endif
/// #endif


In webpack build directory:

npm install ifdef-loader --save-dev


Example of use with TypeScript files, enabling the DEBUG and version variables:

In webpack.config.json put ifdef-loader after ts-loader so that files are processed before going into TypeScript compiler:

// define preprocessor variables
const opts = {
   DEBUG: true,
   version: 3,
   "ifdef-verbose": true,                 // add this for verbose output
   "ifdef-triple-slash": false,           // add this to use double slash comment instead of default triple slash
   "ifdef-fill-with-blanks": true         // add this to remove code with blank spaces instead of "//" comments
   "ifdef-uncomment-prefix": "// #code "  // add this to uncomment code starting with "// #code "

/* ... */ { 
   test: /\.tsx?$/, 
   exclude: /node_modules/, 
   use: [
      { loader: "ts-loader" }, 
      { loader: "ifdef-loader", options: opts } 

// alternatively, options can be passed via query string:
const q = require('querystring').encode(opts);
/* ... */ { 
   test: /\.tsx?$/, 
   exclude: /node_modules/, 
   loaders: [ "ts-loader", `ifdef-loader?${q}` ] 

in example.ts:

/// #if DEBUG
     /* code to be included if DEBUG is defined */
///   #if version <2
        /* code to be included if DEBUG is defined and version < 2*/
///   #endif
/// #endif

Code in comments

Often times writing #if ... #else ... #endif results in code that is not syntactically valid or does not pass the LINT check. A possible workaround is to hide such code in comments and let ifdef-loader uncomment it if it's part of the block that has to be included in the output.


The following code is invalid because the linter sees a double declaration of the a variable.

// #if DEBUG
let a=1;
// #else
let a=2;
// #endif

Using code in comments:

// #if DEBUG
let a=1;
// #else
// #code let a=2;
// #endif

The code is now under comment so it's ignored by the linter; but it's uncommented by ifdef-loader if the else branch has to be included in the output (that is when DEBUG==false).

The // #code string prefix can be changed and has to be explictly specified in the options object:

const opts = {
   // ...
   "ifdef-uncomment-prefix": "// #code ",
   // ...




Contributions in the form of issues or pull requests are welcome.


  • v2.3.0 added option uncomment-prefix to write code in comments allowing it to pass through linters and syntax checking

  • v2.2.0 added option fill-with-blanks for removing code with blank spaces instead of // comments

  • v2.1.0 added support for #elif clause.

  • v2.0.0 BREAKING CHANGE: options are now passed using the standard Webpack API (loader-utils). See below for the upgrade.

  • v1.0.0 changed to triple slash comment syntax. Double slash syntax deprecated and available by turning off the ifdef-triple-slash option.

  • v1.0.3 fixed bug occurring with short lines. Improved handling of line termination (CRLF vs LF) in order to preserve source maps.

  • v1.1.0 added support for #else clauses.

Upgrading from v1 to v2

In v2 options are passed differently than v1, so you need to update your webpack.config.js. Just do the following simple changes:

/* from */ const q = require('querystring').encode({json: JSON.stringify(opts)});
/* to   */ const q = require('querystring').encode(opts);
/* you can keep the  ... `ifdef-loader?${q}` ... syntax    */
/* but it's better to pass options directly (see the docs) */


Webpack loader for JavaScript/TypeScript conditional compilation.






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