Sounds like an evil villain’s superpower, but actually could be a helpful sidekick in your Rails app.
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In a Rails app, you sometimes let users add extra fields to a create/edit form, e.g. when CRUD-ing multiple child records within the parent record’s view.
And sometimes you might want something in the view to change as a result.
And you might not be using jQuery, because perhaps you don’t need a whole library for one or two small JS things.
In this particular situation, MutationObserver could help!
When you set a MutationObserver on a target element, that element can watch for changes to its attributes, data and child nodes and respond as instructed.
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View this simple example’s code here.
Current browser support:
Full syntax & usage info: