Promise analogy

Imagine this scenario: I walk up to the counter at a fast-food restaurant, and place an order for a cheeseburger. I hand the cashier $1.47. By placing my order and paying for it, I've made a request for a value back (the cheeseburger). I've started a transaction.

But often, the cheeseburger is not immediately available for me. The cashier hands me something in place of my cheeseburger: a receipt with an order number on it. This order number is an IOU ("I owe you") promise that ensures that eventually, I should receive my cheeseburger.

So I hold onto my receipt and order number. I know it represents my future cheeseburger, so I don't need to worry about it anymore -- aside from being hungry!

While I wait, I can do other things, like send a text message to a friend that says, "Hey, can you come join me for lunch? I'm going to eat a cheeseburger."

I am reasoning about my future cheeseburger already, even though I don't have it in my hands yet. My brain is able to do this because it's treating the order number as a placeholder for the cheeseburger. The placeholder essentially makes the value time independent. It's a future value.

Eventually, I hear, "Order 113!" and I gleefully walk back up to the counter with receipt in hand. I hand my receipt to the cashier, and I take my cheeseburger in return.

In other words, once my future value was ready, I exchanged my value-promise for the value itself.

But there's another possible outcome. They call my order number, but when I go to retrieve my cheeseburger, the cashier regretfully informs me, "I'm sorry, but we appear to be all out of cheeseburgers." Setting aside the customer frustration of this scenario for a moment, we can see an important characteristic of future values: they can either indicate a success or failure.

Every time I order a cheeseburger, I know that I'll either get a cheeseburger eventually, or I'll get the sad news of the cheeseburger shortage, and I'll have to figure out something else to eat for lunch.

Note: In code, things are not quite as simple, because metaphorically the order number may never be called, in which case we're left indefinitely in an unresolved state. We'll come back to dealing with that case later. - You-Dont-Know-JS

To rephrase this analogy ending: After you exchange the money for the receipt you could have 3 main scenarios:

• Your number gets called, and you get a cheeseburger.
• Your number gets called, and employee informs you they are out of cheeseburgers.
• You never never gets called, they lost your order ticket.