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You can use editor markers to highlight problems inside the editor. Press the "mark vars" button and the clear markers buttons to see, and look in the source code of that page how to use them.
Internals
The editor loads the Ace editor's javascript from a CDN (https://cdnjs.cloudflare.com/ajax/libs/ace/{version}/xxx). This allows easy caching and deployment. To add the required parts as HeaderContributors to a page there are two methods:
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The setMode(String) method can be called to set the editor's mode (which language is being edited), like: editor.setMode("/ace/mode/javascript");
Available methods to play with the editor state
The following methods are implemented: