No. 276 · September 28, 2026 · Gentle

Type-level if

Implement If<Condition, Then, Else> so the literal boolean condition selects exactly one branch.

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Par 2

type-level-if.ts

type If<Condition extends boolean, Then, Else> = ???

type If<Condition extends boolean, Then, Else> = ???

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How a check is judged Exact type equality, not assignability — an intersection is not the same as the flattened object.

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Published September 29, 2026

type If<Condition extends boolean, Then, Else> = Condition extends true ? Then : Else

type If<Condition extends boolean, Then, Else> = boolean extends Condition ? Then : Else

Assignability has a direction. boolean is not assignable to the narrower literal true, so reversing the question selects the wrong branch.

Condition extends true ? Then : Else
With a literal condition the check has one answer, so only one branch survives.

A conditional type is an if whose question is assignability, and the left-to-right direction is part of the meaning.