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;
	
	// switched to Function construction instead of with+eval thanks to @Sayene and @GoldraK
	try {
		// create function returning expression;
		var func = new Function(...Object.keys(context), 'return ' + expression);
		// call function passing context values
		result = func(...Object.values(context));     
	} catch(e) {
		console.warn('•Expression: {{x \'' + expression + '\'}}\n•JS-Error: ', e, '\n•Context: ', context);
	}

// No more `with`, everyone uses `strict mode` these days	 
// Commenting this out, I will leave it in here for those of you who live on the edge.
	 
//    // 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 are 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]);
});

Excellent work.thx.

What's the license to use this? MIT? Thanks for sharing this!

This is wonderful. Thank you.

@bgashler1 MIT it is

This is great. Is there anyway to replace the eval/with statements to avoid a Parser Error when using strict mode?
I've asked the question in SO: https://stackoverflow.com/questions/50827130/extended-if-handlebars-helper-without-with-statement

This is the most legit handlebars helper from what i've seen.

I believe this is working according to the comments below, however, I found it difficult to run it properly on my express application
I have this code:

const express = require("express");
var exphbs = require("express-handlebars");

const app = express();
//this is not working
const hbs = exphbs.create({
  helpers: {
    xif: (expression, options) => {
      return Handlebars.helpers["x"].apply(this, [expression, options])
        ? options.fn(this)
        : options.inverse(this);
    }
  }
});

can some one point me on the right direction please

@edoves xif requires another helper x
Also, it is not recommend to use shorthand function syntax () => {} because the functions will get immediately bound to the context at the time they get created, and this will remain that context, and we don't want that, so i use the long syntax function () {}

const express = require("express");
var exphbs = require("express-handlebars");

const helpers = {
    xif: function (expression, options) {
      return helpers["x"].apply(this, [expression, options]) 
        ? options.fn(this)
        : options.inverse(this);
    },
    x: function (expression, options) {
        var result;
        var context = this;
        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;
    }
};

const app = express();
//this is not working
const hbs = exphbs.create({
    helpers  
});

Thanks, man. I appreciate it. What I really want to achieve is how I'm going to put a comparison of my if statement with handlebars
Here is my template sample:
`

` What I want is to happen is => ` {{#if pageID == "home" }} jumbotron hidden-xs{{/if}}`

this should work

 {{#xif ' pageID == "home" ' }} jumbotron hidden-xs{{/xif}}

notice that the expression is a string, using singlequotes', but inside of it you can either use doublequotes, or single quotes with escaping.

Hi @akhoury fantastic men thank a lot but, seem like the {{ else }} part is not working

Here is my code
{{#xif ' pageID == "home" ' }} col-sm-8 {{else}} col-sm-4 text-center {{/xif}}">

@chrisross to avoid using with() and keep strict mode you may try this approach:

 try {
        let func = new Function(...Object.keys(context), 'return ' + expression); // create function returning expression;
        result = func(...Object.values(context));     // call function passing context values
       } catch(e) {
        console.warn('•Expression: {{x \'' + expression + '\'}}\n•JS-Error: ', e, '\n•Context: ', context);
       }

@Sayene, not a bad idea at all.

@edoves can you create a snippet where the {{else}} isn't working? works fine when i test it, also which Handlebars version are you using, this is a very old helper, things might have changed since

there is one big problom, i cant use @root as a part of an input var...

•Expression: {{x '@root.methodsExist == longname'}}
•JS-Error:  SyntaxError: Invalid or unexpected token
    at Object.<anonymous> (/workspace/Roblox-File-Handler/dont_doc/customBlockHelpers.hbs.js:18:21)
    at Object.<anonymous> (/workspace/Roblox-File-Handler/dont_doc/customBlockHelpers.hbs.js:22:8)
    at Object.<anonymous> (/workspace/Roblox-File-Handler/dont_doc/customBlockHelpers.hbs.js:36:30)
    at Object.wrapper (/workspace/Roblox-File-Handler/node_modules/handlebars/dist/cjs/handlebars/internal/wrapHelper.js:15:19)
    at eval (eval at createFunctionContext (/workspace/Roblox-File-Handler/node_modules/handlebars/dist/cjs/handlebars/compiler/javascript-compiler.js:262:23), <anonymous>:10:130)
    at Object.prog [as fn] (/workspace/Roblox-File-Handler/node_modules/handlebars/dist/cjs/handlebars/runtime.js:268:12)
    at Object.<anonymous> (/workspace/Roblox-File-Handler/node_modules/handlebars/dist/cjs/handlebars/helpers/if.js:29:22)
    at Object.wrapper (/workspace/Roblox-File-Handler/node_modules/handlebars/dist/cjs/handlebars/internal/wrapHelper.js:15:19)
    at eval (eval at createFunctionContext (/workspace/Roblox-File-Handler/node_modules/handlebars/dist/cjs/handlebars/compiler/javascript-compiler.js:262:23), <anonymous>:21:47)
    at prog (/workspace/Roblox-File-Handler/node_modules/handlebars/dist/cjs/handlebars/runtime.js:268:12) 
•Context:  {
  id: 'Instance',
  longname: 'Instance',
  name: 'Instance',
  kind: 'class',
  scope: 'global',
  description: 'Instance is the base class for all classes in the Roblox class hierarchy. Every other class that the Roblox engine defines inherits all of the members of Instance.',
  thisvalue: undefined,
  customTags: [ { tag: 'shortdescription', value: 'The main Baseclass' } ],
  meta: {
    lineno: 9,
    filename: 'Instance.js',
    path: '/workspace/Roblox-File-Handler/Classes'
  },
  order: 48
}

there are alot mor, but i dont thnk you need to see all of it

P.S: Tell me if you need to see code and hbs files...

can you create a code snippet maybe on https://codepen.io to show the error live?

Hi,
I tried the code but received this error

Uncaught (in promise) SyntaxError: Strict mode code may not include a with statement

I show with is deprecated.

do you know other system to work?

Thanks for the time.

yea you have to disabled strict mode to have with working.

I rewrite this functión to delete with and can use strict mode

Handlebars.registerHelper("x", function (expression, options) {
    let result;
    
    // Create a new object to hold variables from the context
    let contextVars = {};
    let context = this;

    // Copy relevant properties from the context to the new object
    for (var key in context) {
        if (context.hasOwnProperty(key) && !(key in options)) { // Avoid overriding options
            contextVars[key] = context[key];
        }
    }

    let contextValues = Object.values(contextVars);
    // Create a function with the context variables exposed as local variables
    var func = new Function(...Object.keys(contextVars), 'return ' + expression);    
    
    try {
        result = func(...contextValues);
    } catch (e) {
        console.warn('•Expression: {{x \'' + expression + '\'}}\n•JS-Error: ', e, '\n•Context: ', context);
    }

    return result;
});

That works too! good idea

@GoldraK it seems that @Sayene did almost the same thing a few years back and I totally forgot! thank you both! Updated the gist to use new Function() instead

//i use your plugin and met some issues there is:
var data ={
test:{
firstName: 'Joan',
age: '21',
email: 'joan@aaa.bbb',
lastName:'check',
},
};
//and template is:

{{#each this}}

{{x "'Hi ' + firstName"}}, {{x 'lastName'}}

{{x '"you were born in " + parseInt(age, 10)'}}

{{@key}}

  {{#xif "@key=='test'"}}
 <span>good</span>
 {{else}}

not good
{{/xif}}
{{#xif 'parseInt(age) >= 21'}} login here:

http://foo.bar?email={{x 'encodeURIComponent(email)'}}

{{else}} Please go back when you grow up. {{/xif}}

{{/each}}

//my issue is i can not access @key inside of xif block, i am really needed to use this variable, {{@key}} this worked well but not in xif block,or i missed some way to do so, can you help me out, thanks in advance!