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Inferable Optional Tuple types are a feature in TypeScript that allows the type of a tuple to be inferred automatically based on the values assigned to it. It also allows for individual elements in the tuple to be set as optional.

For example, let's say we have the following tuple:

const myTuple: [string, number, boolean] = ['hello', 42, true];

If we want to make the third element optional, we can use the inferable optional tuple syntax like this:

const myTuple: [string, number, boolean?] = ['hello', 42];

TypeScript will automatically infer the type of the tuple to be [string, number, boolean?]. The question mark ? next to the boolean type indicates that it is optional. This means that we can create an array with only two elements, and TypeScript will not complain:

const myTuple: [string, number, boolean?] = ['hello', 42];

However, if we try to create an array with four elements, TypeScript will give us an error:

const myTuple: [string, number, boolean?] = ['hello', 42, true, 'hello again'];
// Error: Type 'string' is not assignable to type 'boolean | undefined'.

In summary, inferable optional tuple types are a powerful feature in TypeScript that allow for more flexibility in defining tuples. They allow for easy inference of types and make it easier to work with optional elements in a tuple.