Christopher Ehrlich (@ccccjjjjeeee.bsky.social)

What Does "-?" Mean in TypeScript? Introduced in TypeScript 2.8, if the <code>?</code> modifier is prefixed with <code>-</code> (i.e. <code>-?</code>), it means that the <code>?</code> modifier is removed from a <a href="https://www.typescriptlang.org/docs/handbook/2/mapped-types.html" target="_blank">mapped type</a>, making all properties required. If the <code>?</code> modifier has no <a href="https://www.designcise.com/web/tutorial/what-does-plus-question-mark-mean-in-typescript" target="_blank"><code>+</code></a> or <code>-</code> prefix, then it is the same as if the <code>?</code> modifier was prefixed with <a href="https://www.designcise.com/web/tutorial/what-does-plus-question-mark-mean-in-typescript" target="_blank"><code>+</code></a> (i.e. <a href="https://www.designcise.com/web/tutorial/what-does-plus-question-mark-mean-in-typescript" target="_blank"><code>+?</code></a>). For example, using "<code>-?</code>", the <a href="https://www.typescriptlang.org/docs/handbook/utility-types.html#requiredtype" target="_blank"><code>Required</code> utility type</a> can be implemented like so: <pre> type Required<Type> = { [Property in keyof Type]-?: Type[Property] }; </pre> This maps all keys of type "<code>Type</code>" and makes them required. You can, of course, make use of <code>-?</code> in various other ways. Consider for example the following, where the custom utility type takes all the properties from the type "<code>Type</code>" and changes their values to be required and be of type <code>string</code>: <pre> // TypeScript 2.8+ type RequestBody<Type> = { [Property in keyof Type]-?: string; }; </pre> This could, for instance, be used like so: <pre> // TypeScript 2.8+ type Patient = { name: string; age?: number; recovered?: boolean; }; type PatientRequestBody = RequestBody<Patient>; const patient: PatientRequestBody = { name: 'John Doe', age: '23', recovered: 'false', }; </pre> If any property from the "<code>Patient</code>" type is missing or a value is not of type <code>string</code>, this would throw an error: <pre> // TypeScript 2.8+ // Error: Property 'recovered' is missing in type '{ name: string; age: string; }' but required in type 'RequestBody<Patient>'. const patient: PatientRequestBody = { name: 'John Doe', age: '23', }; </pre> <pre> // TypeScript 2.8+ const patient: PatientRequestBody = { name: 'John Doe', age: '23', // Error: Type 'boolean' is not assignable to type 'string'. recovered: false, }; </pre> https://www.designcise.com/web/tutorial/what-does-minus-question-mark-mean-in-typescript