How do I include external JavaScript libraries in Salesforce lightning components?

I include external JavaScript libraries in Salesforce lightning components by following these steps:

1. Load your external java script library in static resource. Goto >> SetUp>> Custom Code >> Static           Resources

2. Use <ltng:require> tag to load the Java Script library in lightnig component as follows.

Thanks.

Adding some points:

Reference a JavaScript library that you’ve uploaded as a static resource, use a <ltng:require> tag in your .cmp or .app markup.
The framework’s content security policy mandates that external JavaScript libraries must be uploaded to Salesforce static resources. For more information on static resources, see “Static Resources” in the Salesforce online help.

Here’s an example of using <ltng:require>.
<ltng:require scripts=”{!$Resource.resourceName}”
afterScriptsLoaded=”{!c.afterScriptsLoaded}” />

hope this helps you!