JavaScript is a single-threaded programming language. This means that JavaScript can do only one thing at a single point in time.
Inside Browser, there is a Javascript engine (we are considering V8 for chrome.) and an environment to run javascript properly. Javascript engine has two parts, Heap and Call Stack. And the engine has some assistant named Web APIs and Callback Queue.
It's an unstructured memory block. Our code's memory allocation happens here. As a programmer we don't have to worry much about heaps.