• Only one line of JavaScript can run at a time...
• ... but the browser can keep track of multiple events at the same time
• The browser can keep track of multiple timers, network requests, user interactions, etc
• This is why the browser "catches up" after lagging
• Timers, interactions, etc can cause your code to be put on a queue
• If nothing else is happening, your code runs immediately
• If other stuff is happening, the event will be added to the end of the line
• When the browser is idle, it will run things from the front of the queue

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