infer Keyword in Conditional Types

TL;DR

Explanation

infer is used in the extends clause of conditional types: type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never. Here infer R captures the return type. Built-in uses: ReturnType<T>, Parameters<T>, InstanceType<T>, Awaited<T>. Custom uses: extracting element type from arrays (T extends (infer U)[] ? U : never), unwrapping Promises, extracting function argument types. infer can only appear on the right-hand side of extends. Multiple infer in one conditional is supported.

Common Misconception

Why It Matters

Common Mistakes

• Using infer outside conditional types — syntax error.
• Not knowing infer captures the narrowest type possible.
• Forgetting the never fallback — conditional types need both branches.

Code Examples

// Without infer — can't extract return type generically:
type GetReturn<T> = any; // No way to extract without infer

// Built-in pattern:
type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never;

// Custom — unwrap array element type:
type ArrayElement<T> = T extends (infer U)[] ? U : never;
type Item = ArrayElement<string[]>; // string

// Custom — unwrap Promise:
type Unwrap<T> = T extends Promise<infer U> ? U : T;

References

• ↗ https://www.typescriptlang.org/docs/handbook/2/conditional-types.html#inferring-within-conditional-types

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