No. 226 · August 3, 2026 · Gentle

Rebuild Readonly

Implement MyReadonly<T> so every property becomes readonly. One level only — nested objects are left alone.

Try the puzzle yourself

Par 2

Puzzle

rebuild-readonly.ts

interface Config {
  host: string
  port: number
}

type MyReadonly<T> = ???

type MyReadonly<T> = ???

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Replace ??? — your solution is checked against the cases below. Tab indents; press Escape then Tab to move focus out.

Checks

MyReadonly<Config>—
→ { readonly host: string; readonly port: number }
MyReadonly<{ a: string }>—
→ { readonly a: string }
MyReadonly<{}>—
→ {}

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

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Annotated solution

Published August 4, 2026

The solution

type MyReadonly<T> = { readonly [K in keyof T]: T[K] }

The common wrong answer

type MyReadonly<T> = { [K in keyof T]: Readonly<T[K]> }

This applies Readonly to each *value* instead of to the mapping. For a primitive like string that does nothing at all, so the keys stay mutable and the result is just a copy of T.

Line by line

readonly [K in keyof T]
The modifier precedes the key clause, mirroring how you would write readonly host: string in an interface. It marks the property, not the type of its value.
T[K]
The value passes through untouched, which is what keeps this shallow. Recursing here instead is exactly how DeepReadonly is built.

Takeaway

Modifiers act on properties; wrapping the value type is a different operation entirely. Confusing the two produces a type that looks right and enforces nothing.

Uses

Mapped types Readonly Partial Pick Omit Record Mapped types Make Omit catch typos Flatten an intersection Require at least one property Make two prop shapes mutually exclusive