What is key string?

What is the meaning of string keys?

It's an indexer. It defines an indexed property that can be used to access a collection of the object by using objectName["key"] like for example a Dictionary<string,T> .

What is a mapped object type?

A mapped type is a generic type which uses a union of PropertyKey s (frequently created via a keyof ) to iterate through keys to create a type: type OptionsFlags < Type > = { [ Property in keyof Type ]: boolean; };

Is not assignable to type string?

The "Type 'string' is not assignable to type" TypeScript error occurs when we are trying to assign a value of type string to something that expects a different type, e.g. a more specific string literal type or an enum. To solve the error use a const or a type assertion.

Has an any type because expression of type string can't be used to index type?

The error "Element implicitly has an 'any' type because expression of type 'string' can't be used to index type" occurs when we use a string to index an object with specific keys. To solve the error, type the string as one of the object's keys.