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@if and @tap in Dust.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 = '',
//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);
//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(body) {
return chunk.render( bodies.block, context );
else {
_console.log( "Missing body block in the if helper!" );
return chunk;
return chunk.render( bodies['else'], context );
// no condition
else {
_console.log( "No condition given in the if helper!" );
return chunk;
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