Handlebars random JavaScript expression execution, with an IF helper with whatever logical operands and whatever arguments, and few more goodies.

handlebars-helper-x.js

  // for detailed comments and demo, see my SO answer here http://stackoverflow.com/questions/8853396/logical-operator-in-a-handlebars-js-if-conditional/21915381#21915381
  
  /* a helper to execute an IF statement with any expression
    USAGE:
   -- Yes you NEED to properly escape the string literals, or just alternate single and double quotes 
   -- to access any global function or property you should use window.functionName() instead of just functionName()
   -- this example assumes you passed this context to your handlebars template( {name: 'Sam', age: '20' } ), notice age is a string, just for so I can demo parseInt later
   <p>
     {{#xif " name == 'Sam' && age === '12' " }}
       BOOM
     {{else}}
       BAMM
     {{/xif}}
   </p>
   */

  Handlebars.registerHelper("xif", function (expression, options) {
    return Handlebars.helpers["x"].apply(this, [expression, options]) ? options.fn(this) : options.inverse(this);
  });

  /* a helper to execute javascript expressions
   USAGE:
   -- Yes you NEED to properly escape the string literals or just alternate single and double quotes 
   -- to access any global function or property you should use window.functionName() instead of just functionName(), notice how I had to use window.parseInt() instead of parseInt()
   -- this example assumes you passed this context to your handlebars template( {name: 'Sam', age: '20' } )
   <p>Url: {{x " \"hi\" + name + \", \" + window.location.href + \" <---- this is your href,\" + " your Age is:" + window.parseInt(this.age, 10) "}}</p>
   OUTPUT:
   <p>Url: hi Sam, http://example.com <---- this is your href, your Age is: 20</p>
  */
 Handlebars.registerHelper("x", function(expression, options) {
   var result;

   // you can change the context, or merge it with options.data, options.hash
   var context = this;

   // yup, i use 'with' here to expose the context's properties as block variables
   // you don't need to do {{x 'this.age + 2'}}
   // but you can also do {{x 'age + 2'}}
   // HOWEVER including an UNINITIALIZED var in a expression will return undefined as the result.
   with(context) {
     result = (function() {
       try {
         return eval(expression);
       } catch (e) {
         console.warn('•Expression: {{x \'' + expression + '\'}}\n•JS-Error: ', e, '\n•Context: ', context);
       }
     }).call(context); // to make eval's lexical this=context
   }
   return result;
});
  
  /* 
    if you want access upper level scope, this one is slightly different
    the expression is the JOIN of all arguments
    usage: say context data looks like this:
    	
    	// data
    	{name: 'Sam', age: '20', address: { city: 'yomomaz' } }
    	
    	// in template
    	// notice how the expression wrap all the string with quotes, and even the variables
    	// as they will become strings by the time they hit the helper
    	// play with it, you will immediately see the errored expressions and figure it out
    	
    	{{#with address}}
        	{{z '"hi " + "' ../this.name '" + " you live with " + "' city '"' }}
       	{{/with}}
  */
  Handlebars.registerHelper("z", function () {
    var options = arguments[arguments.length - 1]
    delete arguments[arguments.length - 1];
    return Handlebars.helpers["x"].apply(this, [Array.prototype.slice.call(arguments, 0).join(''), options]);
  });
  
  Handlebars.registerHelper("zif", function () {
    var options = arguments[arguments.length - 1]
    delete arguments[arguments.length - 1];
    return Handlebars.helpers["x"].apply(this, [Array.prototype.slice.call(arguments, 0).join(''), options]) ? options.fn(this) : options.inverse(this);
  });
  
  
  
/*
 More goodies since you're reading this gist.
*/

// say you have some utility object with helpful functions which you want to use inside of your handlebars templates

util = {

  // a helper to safely access object properties, think ot as a lite xpath accessor
  // usage: 
  // var greeting = util.prop( { a: { b: { c: { d: 'hi'} } } }, 'a.b.c.d');
  // greeting -> 'hi'
  
  // [IMPORTANT] THIS .prop function is REQUIRED if you want to use the handlebars helpers below, 
  // if you decide to move it somewhere else, update the helpers below accordingly
  prop: function() {
    	if (typeof props == 'string') {
			  props = props.split('.');
		  }
		  if (!props || !props.length) {
			  return obj;
		  }
		  if (!obj || !Object.prototype.hasOwnProperty.call(obj, props[0])) {
			  return null;
		  } else {
			  var newObj = obj[props[0]];
			  props.shift();
			  return util.prop(newObj, props);  
      }
  },
  
  // some more helpers .. just examples, none is required
  isNumber: function(n) {
		return !isNaN(parseFloat(n)) && isFinite(n);
	},
	daysInMonth: function (m, y) {
		y = y || (new Date).getFullYear();
		return /8|3|5|10/.test(m) ? 30 : m == 1 ? (!(y % 4) && y % 100) || !(y % 400) ? 29 : 28 : 31;
	},
	uppercaseFirstLetter: function (str) {
		str || (str = '');
		return str.charAt(0).toUpperCase() + str.slice(1);
	},
	hasNumber: function (n) {
		return !isNaN(parseFloat(n));
	},
	truncate: function (str, len) {
		if (typeof str != 'string') return str;
		len = util.isNumber(len) ? len : 20;
		return str.length <= len ? str : str.substr(0, len - 3) + '...';
	}
};

// a helper to execute any util functions and get its return
// usage: {{u 'truncate' this.title 30}} to truncate the title 
Handlebars.registerHelper('u', function() {
    var key = '';
    var args = Array.prototype.slice.call(arguments, 0);
    
	if (args.length) {
        key = args[0];
        // delete the util[functionName] as the first element in the array
        args.shift();
        // delete the options arguments passed by handlebars, which is the last argument
        args.pop();
    }
	if (util.hasOwnProperty(key)) {
	  // notice the reference to util here
		return typeof util[key] == 'function' ?
			util[key].apply(util, args) :
			util[key];
	} else {
		log.error('util.' + key + ' is not a function nor a property');
	}
});

// a helper to execute any util function as an if helper, 
// that util function should have a boolean return if you want to use this properly
// usage: {{uif 'isNumber' this.age}} {{this.age}} {{else}} this.dob {{/uif}}
Handlebars.registerHelper('uif', function() {
	var options = arguments[arguments.length - 1];
	return Handlebars.helpers['u'].apply(this, arguments) ? options.fn(this) : options.inverse(this);
});

// a helper to execute any global function or get global.property
// say you have some globally accessible metadata i.e 
// window.meta = {account: {state: 'MA', foo: function() { .. }, isBar: function() {...} } }
// usage: 
// {{g 'meta.account.state'}} to print the state

// or will execute a function
// {{g 'meta.account.foo'}} to print whatever foo returns
Handlebars.registerHelper('g', function() {
    var path, value;
	if (arguments.length) {
    path = arguments[0];
    delete arguments[0];
        
    // delete the options arguments passed by handlebars
    delete arguments[arguments.length - 1];
  }
  
  // notice the util.prop is required here  
	value = util.prop(window, path);
	if (typeof value != 'undefined' && value !== null) {
		return typeof value == 'function' ?
			value.apply({}, arguments) :
			value;
	} else {
		log.warn('window.' + path + ' is not a function nor a property');
	}
});

// global if 
// usage: 
// {{gif 'meta.account.isBar'}} // to execute isBar() and behave based on its truthy or not return
// or just check if a property is truthy or not
// {{gif 'meta.account.state'}} State is valid ! {{/gif}}
Handlebars.registerHelper('gif', function() {
	var options = arguments[arguments.length - 1];
	return Handlebars.helpers['g'].apply(this, arguments) ? options.fn(this) : options.inverse(this);
});

// just an {{#each}} warpper to iterate over a global array, 
// usage say you have: window.meta = { data: { countries: [ {name: 'US', code: 1}, {name: 'UK', code: '44'} ... ] } }
// {{geach 'meta.data.countries'}} {{this.code}} {{/geach}}

Handlebars.registerHelper('geach', function(path, options) {
	var value = util.prop(window, arguments[0]);
	if (!_.isArray(value))
		value = [];
	return Handlebars.helpers['each'].apply(this, [value, options]);
});