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The bind directive allows you to bind a property to a DOM element.

This can be a specific attribute, a property of the component class, or a property of the component instance. It may feel redundant to use the bind directive when you can just use curly braces but bind was introduced to solve a very specific problem.

The following is how to do data binding with curly braces:

<input placeholder="{placeholder}">

However, if you want to use a template directly on the HTML file -- when using context providers for example -- you may experience a quick flashing where you see the curly braces instead of the value they are supposed to represent.

For that case and others you may want to use the bind directive.

The following will show the default static value until Javascript kicks in and bind the value to the element.

<span bind="{label}">Default Label</span>
<input bind.placeholder="{placeholder}" placeholder="Default Placeholder">

If you simply do bind="{label}" for example, it is assumed you want to bind to the element text content. You also have the option to specify which property or attribute you want to bind to, like bind.innerHTML="{markup}" or bind.placeholder="{placeholder}"

The bind becomes even more powerful when you server-side-render the HTML. It allows you to set the value in the server and then have the data bound when the javascript loads. It is also a nice way to provide backup for in HTML template rendered in case user has JavaScript disabled.

bind vs attr

The attr will CONDITIONALLY set the attribute, while the bind will ALWAYS set the attribute or property.

You should AVOID using both attr and bind on the same attribute. The last one acting on the element attribute will be the one that ends up binding the attribute value.

<li bind.classname="bind" attr.class="attr, true""attr, true""bind"></li>

<!-- renders:
<li class="attr" id="bind">my item</li> -->

Also, while attr only acts on element attributes, bind acts on the element's property. That's why you use className instead of class when using bind.