[angularjs] What is the best way to conditionally apply attributes in AngularJS?

I need to be able to add for example "contenteditable" to elements, based on a boolean variable on scope.

Example use:

<h1 attrs="{'contenteditable=\"true\"': editMode}">{{content.title}}</h1>

Would result in contenteditable=true being added to the element if $scope.editMode was set to true. Is there some easy way to implement this ng-class like attribute behavior? I'm considering writing a directive and sharing if not.

Edit: I can see that there seems to be some similarities between my proposed attrs directive and ng-bind-attrs, but it was removed in 1.0.0.rc3, why so?

<h1 ng-attr-contenteditable="{{isTrue || undefined }}">{{content.title}}</h1>

will produce when isTrue=true : <h1 contenteditable="true">{{content.title}}</h1>

and when isTrue=false : <h1>{{content.title}}</h1>

In the latest version of Angular (1.1.5), they have included a conditional directive called ngIf. It is different from ngShow and ngHide in that the elements aren't hidden, but not included in the DOM at all. They are very useful for components which are costly to create but aren't used:

<h1 ng-if="editMode" contenteditable=true>{{content.title}}</h1>

Just in case you need solution for Angular 2 then its simple, use property binding like below, e.g. you want to make input read only conditionally, then add in square braces the attrbute followed by = sign and expression.

<input [readonly]="mode=='VIEW'"> 

Regarding the accepted solution, the one posted by Ashley Davis, the method described still prints the attribute in the DOM, regardless of the fact that the value it has been assigned is undefined.

For example, on an input field setup with both an ng-model and a value attribute:

<input type="text" name="myInput" data-ng-attr-value="{{myValue}}" data-ng-model="myModel" />

Regardless of what's behind myValue, the value attribute still gets printed in the DOM, thus, interpreted. Ng-model then, becomes overridden.

A bit unpleasant, but using ng-if does the trick:

<input type="text" name="myInput" data-ng-if="value" data-ng-attr-value="{{myValue}}" data-ng-model="myModel" />
<input type="text" name="myInput" data-ng-if="!value" data-ng-model="myModel" />

I would recommend using a more detailed check inside the ng-if directives :)

You can prefix attributes with ng-attr to eval an Angular expression. When the result of the expressions undefined this removes the value from the attribute.

<a ng-attr-href="{{value || undefined}}">Hello World</a>

Will produce (when value is false)

<a ng-attr-href="{{value || undefined}}" href>Hello World</a>

So don't use false because that will produce the word "false" as the value.

<a ng-attr-href="{{value || false}}" href="false">Hello World</a>

When using this trick in a directive. The attributes for the directive will be false if they are missing a value.

For example, the above would be false.

function post($scope, $el, $attr) {
      var url = $attr['href'] || false;
      alert(url === false);
}

Also you can use an expression like this:

<h1 ng-attr-contenteditable="{{ editMode ? true : false }}"></h1>

To get an attribute to show a specific value based on a boolean check, or be omitted entirely if the boolean check failed, I used the following:

ng-attr-example="{{params.type == 'test' ? 'itWasTest' : undefined }}"

Example usage:

<div ng-attr-class="{{params.type == 'test' ? 'itWasTest' : undefined }}">

Would output <div class="itWasTest"> or <div> based on the value of params.type

I got this working by hard setting the attribute. And controlling the attribute applicability using the boolean value for the attribute.

Here is the code snippet:

<div contenteditable="{{ condition ? 'true' : 'false'}}"></div>

I hope this helps.

Edit: This answer is related to Angular2+! Sorry, I missed the tag!

Original answer:

As for the very simple case when you only want to apply (or set) an attribute if a certain Input value was set, it's as easy as

<my-element [conditionalAttr]="optionalValue || false">

It's the same as:

<my-element [conditionalAttr]="optionalValue ? optionalValue : false">

(So optionalValue is applied if given otherwise the expression is false and attribute is not applied.)

Example: I had the case, where I let apply a color but also arbitrary styles, where the color attribute didn't work as it was already set (even if the @Input() color wasn't given):

@Component({
  selector: "rb-icon",
  styleUrls: ["icon.component.scss"],
  template: "<span class="ic-{{icon}}" [style.color]="color==color" [ngStyle]="styleObj" ></span>",
})
export class IconComponent {
   @Input() icon: string;
   @Input() color: string;
   @Input() styles: string;

   private styleObj: object;
...
}

So, "style.color" was only set, when the color attribute was there, otherwise the color attribute in the "styles" string could be used.

Of course, this could also be achieved with

[style.color]="color" 

and

@Input color: (string | boolean) = false;

Was able to get this working:

ng-attr-aria-current="{{::item.isSelected==true ? 'page' : undefined}}"

The nice thing here is that if item.isSelected is false then the attribute simply isn't rendered.

I actually wrote a patch to do this a few months ago (after someone asked about it in #angularjs on freenode).

It probably won't be merged, but it's very similar to ngClass: https://github.com/angular/angular.js/pull/4269

Whether it gets merged or not, the existing ng-attr-* stuff is probably suitable for your needs (as others have mentioned), although it might be a bit clunkier than the more ngClass-style functionality that you're suggesting.

For input field validation you can do:

<input ng-model="discount" type="number" ng-attr-max="{{discountType == '%' ? 100 : undefined}}">

This will apply the attribute max to 100 only if discountType is defined as %