less.js/lib/less/functions/function-caller.js

var functionCaller = function(name, context, index, currentFileInfo) {
    this.name = name.toLowerCase();
    this.index = index;
    this.context = context;
    this.currentFileInfo = currentFileInfo;

    this.func = context.frames[0].functionRegistry.get(this.name);
};
functionCaller.prototype.isValid = function() {
    return Boolean(this.func);
};
functionCaller.prototype.call = function(args) {
    return this.func.apply(this, args);
};

module.exports = functionCaller;