How to add conditional attribute in Angular 2?
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.
Solution 1:
null removes it:
[attr.checked]="value ? '' : null"
or
[attr.checked]="value ? 'checked' : null"
Hint:
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.
See also What is the difference between properties and attributes in HTML?
What to bind to when there is no such property
Alternatives are [style.xxx]="...", [attr.xxx]="...", [class.xxx]="..." depending on what you try to accomplish.
Because <input> only has a checked attribute, but no checked property [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.
Property-attribute connection
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-forprobably didn't work because it's a keyword in C or JavaScript)Why is colspan not a known native attribute in Angular 2?
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 colspan)
Solution 2:
in angular-2 attribute syntax is
<div [attr.role]="myAriaRole">
Binds attribute role to the result of expression myAriaRole.
so can use like
[attr.role]="myAriaRole ? true: null"
Solution 3:
Refining Günter Zöchbauer answer:
This appears to be different now. I was trying to do this to conditionally apply an href attribute to an anchor tag. You must use undefined for the 'do not apply' case. As an example, I'll demonstrate with a link conditionally having an href attribute applied.
--EDIT--
It looks like angular has changed some things, so null will now work as expected. I've update the example to use null rather than undefined.
An anchor tag without an href attribute becomes plain text, indicating a placeholder for a link, per the hyperlink spec.
For my navigation, I have a list of links, but one of those links represents the current page. I didn't want the current page link to be a link, but still want it to appear in the list (it has some custom styles, but this example is simplified).
<a [attr.href]="currentUrl !== link.url ? link.url : null">
This is cleaner than using two *ngIf's on a span and anchor tag, I think. It's also perfect for adding a disabled attribute to a button.
Solution 4:
If it's an input element you can write something like....
<input type="radio" [checked]="condition"> The value of condition must be true or false.
Also for style attributes...
<h4 [style.color]="'red'">Some text</h4>
Solution 5:
you can use this.
<span [attr.checked]="val? true : false"> </span>