How to add conditional attribute in Angular 2?

Issue

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

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.

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)

Answered By – Günter Zöchbauer

This Answer collected from stackoverflow, is licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0

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