Most Promises I have encountered in code are .then()’d once and then not used again. But, I want to demystify Promises so that we might find more uses for them. I like to think of Promises like a cached value.

When viewing a Promise like a cache, the .then() API gives us access to its cached value. While the Promise’s value is not yet fulfilled, the .then() API makes the caller(s) wait to be called back. If the Promise’s value is already fulfilled, the .then() API will call the callback with the value as soon as possible. Promise objects can be:

Additional demystification: even though Promise is a built-in browser class, Promise instances are not treated as special in any way by the language.

Promise instances are:

• regular objects and can be assigned to variables,
• passed around as parameters in our programs,
• kept long-term for reuse (used as an object cache),
• .then()’d multiple times over time, as well as .then()’d multiple times simultaneously by different asynchronous actors
• garbage collected when they go out of scope like any other JavaScript object would, even when the Promise does not become fulfilled. (In other words, there’s no super-secret internal reference to your Promise that you do not control that can keep it from garbage collecting.)