Promise Chaining

Calling then on a promise does two things. It registers a handler, and it returns a new promise that settles after the handler has run. The next few lines check the second part:

false
true
second fulfilled with 2

second is a different promise from first, and it was fulfilled with 2, the value that the handler returned. Because then returns a promise, you can call then again on the result, and again on that result. A sequence of then and catch calls connected in this way is a promise chain

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