@if and @tap in Dust.js

gistfile1.js

dust.helpers["tap"] = function(input, chunk, context) {
    // return given input if there is no dust reference to resolve
    // dust compiles a string/reference such as {foo} to a function
    if (typeof input !== "function") {
      return input;
    }

    var dustBodyOutput = '',
      returnValue;

    //use chunk render to evaluate output. For simple functions result will be returned from render call,
    //for dust body functions result will be output via callback function
    returnValue = chunk.tap(function(data) {
      dustBodyOutput += data;
      return '';
    }).render(input, context);

    chunk.untap();

    //assume it's a simple function call if return result is not a chunk
    if (returnValue.constructor !== chunk.constructor) {
      //use returnValue as a result of tap
      return returnValue;
    } else if (dustBodyOutput === '') {
      return false;
    } else {
      return dustBodyOutput;
    }
  };

dust.helpers["if"] = function( chunk, context, bodies, params ){
    var body = bodies.block,
        skip = bodies['else'];
    if( params && params.cond){
      var cond = params.cond;
        context.stack.tail.head.index = context.stack.index + 1;
        context.stack.tail.head.of = context.stack.of;
      cond = dust.helpers.tap(cond, chunk, context);
      // eval expressions with given dust references
      if(eval(cond)){
       if(body) {
        return chunk.render( bodies.block, context );
       }
       else {
         _console.log( "Missing body block in the if helper!" );
         return chunk;
       }
      }
      if(skip){
       return chunk.render( bodies['else'], context );
      }
    }
    // no condition
    else {
      _console.log( "No condition given in the if helper!" );
    }
    return chunk;
  };