less.js/test/browser/runner-preProcessorPlugin-options.js

var preProcessor = function() {};

preProcessor.prototype = {
    process : function (src, extra) {
        var injected = '@color: red;\n';
        var ignored = extra.imports.contentsIgnoredChars;
        var fileInfo = extra.fileInfo;
        ignored[fileInfo.filename] = ignored[fileInfo.filename] || 0;
        ignored[fileInfo.filename] += injected.length;
        return injected + src;
    }
};

var preProcessorPlugin = {
    install: function(less, pluginManager) {
        pluginManager.addPreProcessor( new preProcessor());
    }
};

var less = {logLevel: 4,
    errorReporting: "console",
    plugins: [preProcessorPlugin]};