Eye Training · Promises and async/await

Every promise is a question: who waits for it, and who hears if it fails?

Part 2 · Surface 12. Bug counts are hidden and some snippets are clean. See Start Here for how to use these drills.

J · The basics

M · Three years in, these should jump out

S · Where seniors stand out

What your eye checks

Whenever you see a promise, async or await, ask:

1. Is every promise awaited, returned, or deliberately handed off? A promise nobody holds is a failure nobody hears about.
2. Does this callback's caller wait for promises? forEach, filter, map without Promise.all, event emitters and constructors don't.
3. Is a promise used as a value? In an if, a filter or a Boolean(…), a promise is always truthy.
4. Sequential or parallel? Independent awaits in a row are slow; dependent ones (a cursor, an ordering) must stay in a row.
5. What happens when one of many fails? all fails fast, allSettled never fails, any needs one success, race takes the first to settle.
6. How many run at once? Promise.all over a user-sized list has no limit.
7. Is every promise handled from the moment it starts? One started early and awaited later can reject while nobody is listening.
8. Does a .then chain return its inner promises, and is .catch at the end?

Tells

Go deeper

• Promises in Real Code and Async/Await Traps
• Controlling Concurrency: limits, pools and backpressure, with a simulation
• Previous surface: try / catch / finally · Next: Timers