T extends U ? X : Y in practice
Write practical conditional types; learn distribution over unions and how to opt out when needed.
T extends U ? X : Y in practice is a free TypeScript Academy lesson on CoddyKit. This is lesson 1 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the TypeScript Academy learning path, and your progress syncs across the web and the CoddyKit app. The TypeScript Academy course includes 3 lessons in total.
Intro
Goal: Use conditional types to express logic in the type system and understand when they distribute over unions.
- Pattern:
T extends U ? X : Y - Distribution: applies branch per union member
Basic pattern
Conditional types pick a branch based on whether T is assignable to string.
type IsString<T> = T extends string ? true : false
type A = IsString<string>
// ^ true
type B = IsString<number>
// ^ falseDistribution
With a naked T, the check runs per union member: string | number becomes string[] | number[].
type ToArray<T> = T extends any ? T[] : never
type C = ToArray<string | number>
// distributes:
// C is string[] | number[]Opt out
Wrap T in a tuple ([T]) to prevent distribution and treat the union as a whole.
type ToArrayNoDist<T> = [T] extends [any] ? T[] : never
type D = ToArrayNoDist<string | number>
// no distribution:
// D is (string | number)[]Real-world utility
Example of the pattern: For arrays, ElementType uses infer to extract the element; otherwise, it leaves the type as is.
type NonEmptyArray<T> = T[] extends [] ? never : T[]
// Usually better:
// type ElementType<T> = T extends ReadonlyArray<infer U> ? U : T
type E1 = ElementType<string[]>
// ^ string
type E2 = ElementType<readonly number[]>
// ^ number
type E3 = ElementType<string>
// ^ stringTips
Tips: Keep conditions simple; note the deployment; reuse helper types; include test files for complex combinations.
// Tips
// - Keep conditions simple; deep nesting harms readability
// - Document distribution behavior for teammates
// - Prefer helper aliases (ElementType, Awaited) for reuse
// - Watch for De Morgan like logic when composing unionsDistribution check
Quick check: When do conditional types distribute?
Recap
Recap: Conditional types model logic in types. Distribution applies per union member; use tuple wrap to opt out.
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Frequently Asked Questions
Is the “T extends U ? X : Y in practice” lesson free?
Yes — the full text of “T extends U ? X : Y in practice” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the TypeScript Academy course, upgrade to CoddyKit PRO. The TypeScript Academy course includes 3 lessons in total.
What will I learn in “T extends U ? X : Y in practice”?
Write practical conditional types; learn distribution over unions and how to opt out when needed. You practise TypeScript Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start TypeScript Academy?
No prior experience is required. TypeScript Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 1 of 3.
How long does the “T extends U ? X : Y in practice” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this TypeScript Academy lesson?
Yes. Every TypeScript Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- T extends U ? X : Y in practice
- infer for ReturnType-like utilities
- Built-ins: ReturnType, Parameters, InstanceType, etc.