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Angular error NG2009 means a component using ViewEncapsulation.ShadowDom has a selector that does not meet custom element tag-name requirements. Rename the selector so it starts with a lowercase letter, contains a hyphen, and uses lowercase characters—for example, change comp to app-comp.
What NG2009 means
Angular classifies NG2009 as a compiler error titled “Invalid Shadow DOM selector.” It occurs when a component configured with ViewEncapsulation.ShadowDom uses a selector that is not a valid custom element tag name. Angular’s error page describes the issue as a selector that “doesn’t match the custom element tag name requirements.” Angular’s NG2009 documentation shows comp as an invalid selector and app-comp as the corrected version.
How to fix NG2009
- Find the component whose decorator sets
encapsulation: ViewEncapsulation.ShadowDom. - Check the component’s
selectorvalue. - Rename the selector to start with a lowercase letter, include a hyphen, and use lowercase characters. For example:
@Component({ selector: 'app-comp', encapsulation: ViewEncapsulation.ShadowDom }) - Run the Angular build or compiler again and confirm that NG2009 is gone.
Why the selector needs a hyphen
Shadow DOM components use custom element tag names, and Angular requires the selector to follow those naming rules. A simple name such as comp does not qualify because it has no hyphen. A name such as app-comp meets the documented requirements.
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If NG2009 remains
- Verify that you changed the selector on the component that actually uses
ViewEncapsulation.ShadowDom, rather than another component. - Check the final selector value for a lowercase starting letter and a hyphen.
- Consult Angular’s Error Encyclopedia if the compiler reports a different or additional error after the rename.
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