Polymer ready vs connectedCallback

Question

I'm wondering what is is the importance of ready() method in polymer and what is the meaning of on-time initialization for such method and what is the differences between read() and connectedCallback() ???

Answer 1

The following should help.

connectedCallback can be called more than once

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Generally all setup work in connectedCallback should be balanced with associated teardown work in disconnectedCallback; if not it usually means connectedCallback is the wrong place to do it

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At this point it's hard to contrive general a use case where ready is the only/best place to do work, although they do come up from time to time in very specific cases.

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ready is called only once, so work done here based on a property value may become invalidated if the property value is changed at a later point by the user

See https://github.com/Polymer/polymer/issues/4108#issuecomment-258983943