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I found the following in a TypeScript declaration file for Angular:
interface IDirective{ compile?: (templateElement: IAugmentedJQuery, What does the ?: after compile do?
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TypeScript 3.7 added support for the ?? operator, which is known as the nullish coalescing operator. We can use this operator to provide a fallback value for a value that might be null or undefined .
The question mark ? in typescript is used in two ways: To mention that a particular variable is optional. To pre-check if a member variable is present for an object.
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.
See: Walkthrough: Interfaces | TypeScript :: Describing Simple Types.
Basically, ? marks the member as being optional in the interface.
(EDIT: As noted in comments, this is not restricted to interfaces.)
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In this case, the ?: is not a single operator, but rather two operators:
? (optional),
: (specify type).
In other languages/cases, ?: would be the Elvis Operator.
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