How to check if an object is a Promise?
Here's a concise way to identify if an object is a Promise. We're relying on the presence of a then method (a classic indicator) or the instanceof keyword:
This mnemonic covers both structural and inheritance-based tests in one fell swoop!
Promise-proof your checks with optional chaining
In the brave new world where optional chaining has your back, you can fortify against unexpected null or undefined values:
It's like adding training wheels to your object checks to prevent crashes! 🚴♀️
Non-standard Promises: "Thenables"
In situations where you're wrestling with non-native or shimmed Promises, look out for "thenable" objects:
This approach recognizes anything with a then method. Useful when dealing with the Promises/A+ specification in the wild!
The shady side of Promise.resolve
Beware rookie mistakes when using Promise.resolve:
Promise.resolvecan be deceptive. When fed a promise-like object, it does not return the original object but births a brand-new Promise.- In case of constructor-created promises,
Promise.resolvemight play a swap game, not returning the same instance.
Mind your steps, explorers!
Tricky situations and pro tips
- instanceof: Not always reliable for Promises from parallel universes (other realms like iframes etc.).
- Cross realms: Promises from another realm or window might flunk this test as they use a different Promise constructor.
- Assorted libraries: Promises from custom libraries might not pass this test. They like to march to their own drum, you know!
Why algorithmic checks rule
- Compatibility: Brings different Promise implementations under one umbrella.
- Uniformity: Abides by the general traits of Promises rather than strict structure.
Top use-cases for developers
Code systematically to handle Promises from all sorts of origins. This is especially relevant if you're building libraries or solutions aiming at multiple JavaScript environments.
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