Adding and removing modifiers in TypeScript

A mapped type can add or strip readonly and optional.

interface Step {
  readonly idx: number;
  title: string;
  stars?: number;
}

type Mutable<T> = { -readonly [K in keyof T]: T[K] };
type AllOptional<T> = { [K in keyof T]+?: T[K] };
type AllRequired<T> = { [K in keyof T]-?: T[K] };

const mutable: Mutable<Step> = { idx: 0, title: "Mapped types" };
mutable.idx = 1; // allowed: readonly was stripped

const patch: AllOptional<Step> = {};
const complete: AllRequired<Step> = { idx: 2, title: "Modifiers", stars: 3 };

console.log(mutable.idx, Object.keys(patch).length, complete.stars);

How it works

1. +? adds optional, -? removes it.
2. -readonly strips the modifier the source had.
3. That is exactly how Partial and Required are defined.

Keywords and builtins used here

• AllOptional
• AllRequired
• Mutable
• Object
• T
• const
• interface
• number
• readonly
• string
• type

The run, in numbers

Lines

17

Characters to type

530

Tokens

163

Three-star pace

110 tpm

At the three-star pace of 110 tokens a minute, this run takes about 89 seconds.

Type this snippet

Step 3 of 3 in Mapped types, step 6 of 12 in Type-level programming.

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