chain is a global variable. It starts with an empty promise. First call to enqueue changes the (global)value of chain to emptyPromise.then(firstJob), second call to enqueue changes it to emptyPromise.then(firstJob).then(secondJob) and so on.

JavaScript promises are objects with a resolver function and an internal asynchronous computation. At some point in the future, the asynchronous computation will complete, and at that point the promise will call its resolver function with the return value of the computation.

`prom.then(fn)` creates a new promise. The new promise’s resolver function is the `fn` inside `then(fn)`, and the new promise’s asynchronous computation is the original promise’s resolver function.