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JS: AngularJS Directive Attribute Binding Explanation

AngularJS Directive Attribute Binding Explanation

When using directives, you often need to pass parameters to the directive. This can be done in several ways. The first 3 can be used whether scope is true or false. This is still a WIP, so validate for yourself.

  1. Raw Attribute Strings

    <div my-directive="some string" another-param="another string"></div>
    
    var directiveFunction = function(){
    	return {
    		link: function(scope, element, attributes){
    
    			console.log(attributes.myDirective);
    			console.log(attributes.anotherParam);
    
    		}
    	};
    }
  2. Observing Interpolated Strings (Also Watches for Changes)

    <div my-directive="{{some string}}" another-param="another string"></div>
    
    var directiveFunction = function(){
    	return {
    		link: function(scope, element, attributes){
    		
    			console.log(attributes.myDirective); //literal string "{{some string}}", no interpolation
    			console.log(attributes.anotherParam); //literally "another string"
    
    			attributes.$observe('myDirective', function(value){
    				console.log(value);
    			});
    
    			attributes.$observe('anotherParam', function(value){
    				console.log(value);
    			});
    
    		}
    	};
    }
  3. Observing Expression Strings (Also watches for changes)

    <div my-directive="modelObject" another-param="modelObject.obj"></div>
    
    var directiveFunction = function(){
    	return {
    		link: function(scope, element, attributes){
    		
    			console.log(attributes.anotherParam); //literally "modelObject.obj"
    		
    			//modelObject is a scope property of the parent/current scope
    			scope.$watch(attributes.myDirective, function(value){
    				console.log(value);
    			});
    			
    			//modelObject.obj is also a scope property of the parent/current scope
    			scope.$watch(attributes.anotherParam, function (value){
    				console.log(value);
    			});
    			
    			//if you tried to use $observe, you would get the literal expression "modelObject" and "modelObject.obj", because there's nothing to interpolate
    
    		}
    	};
    }
  4. Evaluating Object Expressions

    <div my-directive="{ param: 34, param2: 'cool' }" another-param="another string"></div>
    
    var directiveFunction = function(){
    	return {
    		link: function(scope, element, attributes){
    
    			//this is designed for directive configuration if there's a alot of configuration parameters
    			//one can combine this with interpolation, if the configuration is a JSON string
    			var obj = scope.$eval(attributes.myDirective);
    			//can also fallback as a string
    			var string = scope.$eval(attributes.anotherParam);
    
    			console.log(obj);
    			console.log(string);
    
    		}
    	};
    }
  5. Isolate Scope One Way String Binding (Parent changes affect child, child changes does not affect parent)

    <div my-directive="{{some.string}}" another-param="{{another.string}}" string-param="somestring"></div>
    
    var directiveFunction = function(){
    	return {
    		scope: {
    			myDirective: '@',
    			anotherParam: '@',
    			stringParam: '@'
    		},
    		link: function(scope, element, attributes){
    
    			//the '@' binding automatically interpolates the "{{}}" if they exist in the attributes
    			console.log(scope.myDirective); //interpolated string
    			console.log(scope.anotherParam); //interpolated string
    			console.log(scope.stringParam); //literal string
    			
    			//IMPORTANT: if scope.some.string was not defined on the parent scope, then '@' interpolates it into an EMPTY string, it is still a STRING type
    			//if the DOM attribute was not defined, scope.property would returned undefined
    
    			//if the strings are not yet processed when this directive runs
    			//this means they are interpolated, but perhaps the true value has not arrived, such as content from AJAX
    			//see: http://stackoverflow.com/a/14907826/582917
    			attributes.$observe('myDirective', function(value){
    				if(value){
    					console.log(value);
    				}
    			});
    			attributes.$observe('anotherParam', function(value){
    				if(value){
    					console.log(value);
    				}
    			});
    
    			//the $watch method also works because the '@' binding already does the interpolation
    			//see: http://stackoverflow.com/a/12976630/582917 & http://stackoverflow.com/a/17224886/582917
    			//this only works because it's in an isolate scope, therefore myDirective is part of the scope
    			scope.$watch('myDirective', function(value){
    				if(value){
    					console.log(value);
    				}
    			});
    			
    			//the difference between $observe and $watch in this context, is that $observe reruns the interpolation (from the literal attribute value) and watches the value change, whereas the $watch relies on the '@' interpolation and parent binding and simply watches the value change in this current isolated scope
    			//this means if want to react to the value change coming from this directive, you'll need to use $watch, otherwise if you only want to react to parent changes, you may use $observe
    			
    			//multivalue value watching, only possible with $watch
    			//make sure to assign an object outside of the scope
    			//if you decide to return an object "return { }", a new object reference would be created
    			//inside the watch, and thus trigger another watch, resulting in digest errors
    			//MAKE SURE TO DO DEEP WATCH!
    			var multiValues = {};
    			scope.$watch(function(){
    				multiValues.myDirective = scope.myDirective;
    				multiValues.anotherParam = scope.anotherParam;
    				return multiValues;
    			}, function(value){
    				if(value){
    					console.log(value);
    				}
    			}, true);
    
    		}
    	};
    }
  6. Isolate Scope Two Way Object Binding (Parent changes affect child and child changes affect parent)

    <div my-directive="modelObject" another-param="{ thisWill: 'result in digest errors' }"></div>
    
    var directiveFunction = function(){
    	return {
    		scope: {
    			myDirective: '=', //use =? for optionality
    			anotherParam: '='
    		},
    		link: function(scope, element, attributes){
    
    			//modelObject needs to be defined on the parent scope or you can use "=?" for optionality
    			console.log(scope.myDirective); //this will be the parent/current scope's value, it's not the literal string
    			//this will result in digest errors: http://stackoverflow.com/q/13594732/582917
    			//so don't use object expressions with '='
    			console.log(scope.anotherParam);
    			
    			//IMPORTANT: if scope.modelObject was not defined on the parent scope, then '=' interpolates it into an UNDEFINED type, this works for child objects as well like scope.modelObject.childObject
    			//if the DOM attribute was not defined, scope.property would returned undefined
    
    			//the $watch method works when the model values have not yet been processed by the time this directive runs
    			scope.$watch('myDirective', function(value){
    				if(value){
    					console.log(value);
    				}
    			});
    			
    			//$observe is useless here, as there is no interpolation whatsoever, the $watch will react to changes from both parent scope and current scope, furthermore any change to the value here, changes the parent value as well
    			
    			//=? is required if you want to assign values to the current isolated scope property, but the DOM attribute or parent scope property was never defined, checking if it exists or logging its current value won't result in an error
    
    		}
    	};
    }
  7. Isolate Scope Object & Object Literal Expression Binding

    <div my-directive="{ param: 34, param2: 'cool' }" another-param="parentScopeObject"></div>
    
    var directiveFunction = function(){
    	return {
    		scope: {
    			myDirective: '&',
    			anotherParam: '&'
    		},
    		link: function(scope, element, attributes){
    
    			//this will return the actual object from the object expression!
    			console.log(scope.myDirective());
    			//this will return the actual object from the parent scope, if it exists of course!
    			//and no "parentScopeObject" is not a function, it's an object
    			console.log(scope.anotherParam());
    
    		}
    	};
    }
  8. Isolate Scope Function Expression Binding

    <div my-directive="parentScopeFunction(funcParam, secondParam)"></div>
    
    var directiveFunction = function(){
    	return {
    		template: '<button ng-click="myDirective({funcParam: 'blah blah', secondParam: 'blah blah'})">It can be executed from inside the DOM too!</button>',
    		scope: {
    			myDirective: '&'
    		},
    		link: function(scope, element, attributes){
    		
    			//IMPORTANT: if scope.parentScopeFunction was not defined on the parent scope, then '&' interpolates it into a NOOP function, it is still a FUNCTION type
    			//if the DOM attribute was not defined, scope.property would also still return a noop function
    
    			//if it's defined as something other than a function, an error occurs!
    			
    			//parameters passed into the bound function expression must be in the form of an object map
    			scope.myDirective(
    				{
    					funcParam: 'This is the value that is going to be passed in as the funcParam',
    					secondParam: 'This is another param!'
    				}
    			);
    
    		}
    	};
    }
  9. Non-Isolate Scope Function Expression Binding

    <div my-directive="parentScopeFunction(funcParam)" another-func="parentScopeFunction2()"></div>
    
    var directiveFunction = function($parse){
    	return {
    		link: function(scope, element, attributes){
    
    			//see this: 
    			//http://stackoverflow.com/questions/17583004/call-an-angularjs-controller-function-from-a-directive-without-isolated-scope
    			//http://stackoverflow.com/questions/14858682/angularjs-directive-with-isolate-scope-do-i-really-have-to-call-parent-everyw
    			
    			//apply and eval
    			scope.$apply(function() { 
    				scope.$eval(attributes.anotherFunc); 
    			});
    
    			//apply only
    			scope.$apply(attributes.anotherFunc);
    			
    			//$parse method, this allows parameters to be passed
    			var invoker = $parse(attributes.myDirective);
    			invoker(scope, {funcParam: 'example value'});
    
    		}
    	};
    }
@aureustaurus
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aureustaurus commented Feb 4, 2016

Thanks! very helpful!

@bastianwegge
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bastianwegge commented Feb 8, 2016

great stuff !

@taotau
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taotau commented Feb 24, 2016

Angular binding in a nutshell. Thanks for the lightbulb moment!

@chacaldev
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chacaldev commented Feb 26, 2016

Thanks!!! Really helpful!

@paladinwitzy
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paladinwitzy commented Mar 24, 2016

Very helpful, thanks for sharing.

@anilsingh581
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anilsingh581 commented Mar 26, 2016

It might help you for Attribute Binding in Angular
http://www.code-sample.com/2016/03/auto-capitalize-input-char-using.html

@AndreyUtka
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AndreyUtka commented Apr 7, 2016

What about one way expression bindings? ::item

@laranicolas
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laranicolas commented Apr 7, 2016

Thanks for sharing!

@jamesjryan
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jamesjryan commented Apr 18, 2016

Sending more thanks your way.

@Chima1707
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Chima1707 commented Apr 23, 2016

Very helpful

@CMCDragonkai
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CMCDragonkai commented Jun 2, 2016

Thanks for all the feedback. This gist was written 2 years ago, and things may have changed in Angular in the mean time. Though a lot probably still applies. If any of you improve on it, feel free to fork it and put it up, or suggest a diff to apply to the gist above, and I'll update it, but I'm not able to verify whether any new changes works.

@CMCDragonkai
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CMCDragonkai commented Jun 14, 2016

By the way, I've worked on a project callee SnapSearch (https://snapsearch.io/) that allowd sites built in AngularJS and any Javascript framework to be indexed by search engines and social network robots. Might be useful to people frequenting this gist.

@saucey
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saucey commented Aug 6, 2016

good stuff

@gilderlan
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gilderlan commented Aug 29, 2016

\o/

@AndrewIsh
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AndrewIsh commented Sep 20, 2016

Excellent resource, thanks. Any chance you could break the comments? Having to scroll horizontally to read them is a PITA :)

@abhishekbhalani
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abhishekbhalani commented Oct 22, 2016

Good stuff.. thank you 💯

@mdeggies
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mdeggies commented Nov 6, 2016

Saved my butt. Thank you!

@NithinBiliya
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NithinBiliya commented Dec 22, 2016

great material!

@bb-swapnil
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bb-swapnil commented Mar 21, 2017

could not stop myself to say ' Its AWESOME" .... !!!!

@sserbest
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sserbest commented Mar 28, 2017

Very helpful, thanks.

@TSkills
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TSkills commented Apr 12, 2017

Nice nice nice !!

@wuxicn
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wuxicn commented Apr 20, 2017

Awesome! Thank you!

@BurakAkyildiz
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BurakAkyildiz commented Jun 14, 2017

Non isolated binding was helpfull thnx..

@avrim-tr
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avrim-tr commented Aug 4, 2017

HI,

In Example 2,
console.log(attributes.myDirective) WILL BE ITERPULATED

@TatyanaDubrova
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TatyanaDubrova commented Nov 27, 2017

Cool explanation, thank you so much

@fantasiiio
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fantasiiio commented Mar 1, 2018

Thank you very much !

@milad1367
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milad1367 commented May 22, 2018

Thanks, man!

@RobertRajcool
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RobertRajcool commented Jul 23, 2018

Great article

@james-jhang
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james-jhang commented Oct 7, 2018

Thank you

@NewEXE
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NewEXE commented Nov 11, 2020

Thanks from 2020 :-)

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