Power-Up Client Library

When you include the power-up.js or power-up.min.js script on your page, it will put an object on the window called TrelloPowerUp. You can initialize the Power-Up communication library with either:

On your index connection page, where you register your callback functions for each of the capabilities that your Power-Up supports

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window.TrelloPowerUp.initialize({
  'board-buttons': function (t, opts) { ... },
  'card-buttons': function (t, opts) { ... },
  ...
});

Alternatively, your Power-Up may have other pages that are hosted in iframes inside of Trello such as for popups, or for attachment-sections

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var t = window.TrelloPowerUp.iframe();

Once you have access to t (you can call it whatever you want) as in the capability callback functions, or in your secondary iframes, you can use t to ask questions of Trello, as well as have Trello do things for you.