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Traditional JavaScript and Node.js API will utilize the concept of asynchronous callbacks for virtually every input/output operation. It is a common pattern for Node.js API. Whether it's reading a disk file, accessing a database table, or calling a web service, the results are not directly returned to the caller; instead, they are passed into a separate function referred to as a callback.

Because of JavaScript's design, the default way to "freeze" a computation and have the "rest of it" execute later (asynchronously) is to put "the rest of it" inside of a callback function.

For certain types of applications, this can be quite useful and performant; however, it also creates the problem more commonly known as Callback Hell. A lot of code ends up looking like this:

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