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The `hasOwnProperty` check in `_upgradeProperty` fails if a property is only defined through its getter/setter

See original GitHub issue

Hello,

I’ve been trying to implement the lazy properties pattern in a Custom Element I’m working on.

I’m using the _upgradeProperty function from <howto-checkbox>, and I noticed that the hasOwnProperty check returns false for properties that are defined only through getters/setters.

Am I missing something about the way the lazy properties pattern works?

Issue Analytics

State:
Created 6 years ago
Comments:5 (3 by maintainers)

Top GitHub Comments

1reaction

robdodsoncommented, Sep 4, 2017

Have you tried creating the contents of your Shadow DOM in the element’s constructor instead? Here’s an example: https://jsbin.com/dahokeb/2/edit?html,output

The order of lifecycle callbacks should go:

constructor
attributeChangedCallback
connectedCallback

1reaction

surmacommented, Sep 3, 2017

The lazy property pattern is supposed to cover a very specific corner case of custom elements. Here’s the scenario:

<!doctype html>
<my-element id="a"></my-element>
<script src="someFramework.js"></script>
<script src="my-element.js"></script>

Notice how someFramework.js is included (and executed!) before my-element.js, which contains the definition for <my-element>. While someFramework.js executes, #a will be a generic HTMLElement and not MyElement, as the element’s definition has not been loaded yet. If, during that initial execution, someFramework.js sets property x on #a, the DOM object will have a property x. If x is a setter on MyElement, the element needs to ingest that property on its DOM object and trigger the setter manually to not lose the value that has been set. And that’s exactly what _upgradeProperty does.