A Promise.all Analogy

August 15, 2017

I've been splitting time this year in-between MI and MA, and as a result, I've spent a lot of time driving on I-90. Yesterday, my wife and I stopped at a service plaza in the NY stretch of I-90, and this analogy for Promise.all came to mind.

The car stops, and we decide to split up tasks to get back on the road as soon as possible. One of us will pump gas, while the other one will run inside to order Roy Rogers' (at least if you stop in Indian Castle). Here's what that Promise looks like:

Promise.all([pumpGas(), orderFood()])

This Promise is pending until both tasks are completed. Since each action takes an indefinite amount of time, we must wait for both to be completed before moving on. If both tasks are completed successfully, both Promises are resolved, and .then() is called:

Promise.all([pumpGas(), orderFood()]).then(([gas, food]) => {
  /* Now we can drive and eat */

If either or both of us fail to complete our tasks, the Promise is rejected, and .catch() is called:

Promise.all([pumpGas(), orderFood()]).catch(error => {
  /* I forgot my wallet in the car */