Lesson 41 of 55
15 mins readThe infer Keyword in Conditional Types
In Plain English
The infer keyword allows you to introduce a new type variable within the true branch of a conditional type, automatically capturing return types, promise unwraps, or array elements.
Core Rules to Remember
infer Variable: Deduces types in-place within the condition extends clause.
Unwrapping: Commonly used to unwrap Promise<T> (Awaited) or extract function return types.
Live Interactive Example
Hit Run Code to see it liveUnwrapping Return Types and Promises
Python 3.12
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Output Console
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How it works: infer R deduces the exact return type of the function and binds it to R.
Your Turn: Micro Challenge
No pressure! Edit the starter code below and test your solution with instant feedback.
Micro Exercise
Extract First Array Element Type
- Define type First<T> = T extends [infer Head, ...any[]] ? Head : never.
- Test with type F = First<[string, number, boolean]>.
- Assign const item: F = "first_token" and print item.
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Sandbox Output
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