How can I conditionally add an element attribute e.g. the
checked of a checkbox?
Previous versions of Angular had
NgAttr and I think
NgChecked which all seem to provide the functionality that I’m after. However, these attributes do not appear to exist in Angular 2 and I see no other way of providing this functionality.
null removes it:
[attr.checked]="value ? '' : null"
[attr.checked]="value ? 'checked' : null"
Attribute vs property
When the HTML element where you add this binding does not have a property with the name used in the binding (
checked in this case) and also no Angular component or directive is applied to the same element that has an
@Input() checked;, then
[xxx]="..." can not be used.
What to bind to when there is no such property
[class.xxx]="..." depending on what you try to accomplish.
<input> only has a
checked attribute, but no
[attr.checked]="..." is the right way for this specific case.
Attributes can only handle string values
A common pitfall is also that for
[attr.xxx]="..." bindings the value (
...) is always stringified. Only properties and
@Input()s can receive other value types like boolean, number, object, …
Most properties and attributes of elements are connected and have the same name.
When bound to the attribute the property also only receives the stringified value from the attribute.
When bound to the property the property receives the value bound to it (boolean, number, object, …) and the attribute again the stringified value.
Two cases where attribute and property names do not match.
- Error while adding "for" attribute to label in angular 2.0 template
https://developer.mozilla.org/en-US/docs/Web/API/HTMLLabelElement/htmlFor (see the first sentence of the description
htmlFor property reflects the value of the for–
Angular was changed since then and knows about these special cases and handles them so that you can bind to
<label [for]=" even though no such property exists (same for
Answered By – Günter Zöchbauer