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If I have a parent lightning component and the component has ltng:require do I need to require the same libraries from the children components or will the inclusion be propagated to children?

For example the parent lightning component has the following tag inside it:

<ltng:require scripts="{!join(',', $Resource.fooLib1, $Resource.fooLib2)}"/>

If I want to be able to access the fooLib1 in the child component do I need have the following tag in the child:

<ltng:require scripts="{!$Resource.fooLib1}"/>

?

Are there any performance difference? Will the Lightning optimize such inclusions and include the libraries only once (in case the propagation happens in Lightning and in case it is present)?

I tried to google that, but found nothing about it. Will be grateful for any information source provided.

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The documentation doesn't specify, as far as I can tell, but both of your statements are currently correct: a library loaded in a parent component is also available in the child component, and a library included twice is loaded only once. However, since the documentation makes no claims about how SecureWindow exposes these libraries, the optimal solution is to load a library in each component that needs it. There's no real performance penalty, since the library is loaded only once, and you would rather have the library be available than not.

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  • How did you check that the library is loaded only once? Did you use Chrome Dev Tools somehow?
    – manymanymore
    Commented Jun 13, 2018 at 15:34
  • @YaroslavTrofimov Yes, I used the Network tab to verify that the server was queried only once for that file.
    – sfdcfox ♦
    Commented Jun 13, 2018 at 15:36

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