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I came across this line in the type definitions for prop-types:
export type ValidationMap<T> = { [K in keyof T]-?: Validator<T[K]> }; Without - it would be a pretty standard partial mapped type, but I can't find anywhere in the docs where it talks about -?.
Can anyone explain what -? means?
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What does ?: mean in TypeScript? Using a question mark followed by a colon ( ?: ) means a property is optional. That said, a property can either have a value based on the type defined or its value can be undefined .
What is the TypeScript exclamation mark? The non-null assertion operator tells the TypeScript compiler that a value typed as optional cannot be null or undefined . For example, if we define a variable as possibly a string or undefined, the !
The Angular safe navigation operator (?.) is a fluent and convenient way to guard against null and undefined values in property paths.
+ or - allows control over the mapped type modifier (? or readonly). -? means must be all present, aka it removes optionality (?) e.g.:
type T = { a: string b?: string } // Note b is optional const sameAsT: { [K in keyof T]: string } = { a: 'asdf', // a is required } // Note a became optional const canBeNotPresent: { [K in keyof T]?: string } = { } // Note b became required const mustBePreset: { [K in keyof T]-?: string } = { a: 'asdf', b: 'asdf' // b became required } I did a lesson on these mapped type modifiers : https://www.youtube.com/watch?v=0zgWo_gnzVI 🌹
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